I'm having the same case too with 2.6.32 and an intel 855GME. It worked with a fresh installed lenny,
and started to break once I upgraded to sid.
If I disable KMS in /etc/modprobe.d/i915-kms.conf
gdm hangs . If I enable it then it complains LVDS1 is disconnected and I can only
use an external crt connected to my VGA output (with artifacts, but I haven't looked into that).

dmesg says something weird:
[ 22.545367] i801_smbus 0000:00:1f.3: PCI INT B -> Link[LNKB] -> GSI 10 (level, low) -> IRQ 10
[ 22.580061] ACPI: I/O resource 0000:00:1f.3 [0x1880-0x189f] conflicts with ACPI region SBUS [0x1880-0x188f]
[ 22.614495] ACPI: If an ACPI driver is available for this device, you should use it instead of the native driver
[ 22.652403] Marking TSC unstable due to TSC halts in idle

Xorg seems unable to find the laptop LCD through edid :
(--) intel(0): Chipset: "852GM/855GM"
(II) intel(0): Output VGA1 has no monitor section
(II) intel(0): Output LVDS1 has no monitor section
(II) intel(0): found backlight control interface /sys/class/backlight/acpi_video0
(II) intel(0): EDID for output VGA1
(II) intel(0): Manufacturer: GSM Model: 3ad7 Serial#: 16843009
(II) intel(0): Year: 1999 Week: 17
(II) intel(0): EDID Version: 1.1
[...]
(II) intel(0): Modeline "640x480"x59.9 25.18 640 656 752 800 480 489 492 525 -hsync -vsync (31.5 kHz)
(II) intel(0): Modeline "640x480"x59.9 25.17 640 656 752 800 480 490 492 525 -hsync -vsync (31.5 kHz)
(II) intel(0): Modeline "720x400"x70.1 28.32 720 738 846 900 400 412 414 449 -hsync +vsync (31.5 kHz)
(II) intel(0): EDID for output LVDS1
(II) intel(0): Output VGA1 connected
(II) intel(0): Output LVDS1 disconnected
(II) intel(0): Using exact sizes for initial modes
(II) intel(0): Output VGA1 using initial mode 1024x768

If I don't connect the external monitor then it simply says both VGA1 and LVDS1 are disconnected
and hangs. In any case I can't switch vts with Ctrl-Alt-F1 etc.

ideafix:~# i2cdetect -l
ideafix:~# sensors-detect
# sensors-detect revision 5818 (2010-01-18 17:22:07 +0100)

This program will help you determine which kernel modules you need
to load to use lm_sensors most effectively. It is generally safe
and recommended to accept the default answers to all questions,
unless you know what you're doing.

Some south bridges, CPUs or memory controllers contain embedded sensors.
Do you want to scan for them? This is totally safe. (YES/no):
Silicon Integrated Systems SIS5595... No
VIA VT82C686 Integrated Sensors... No
VIA VT8231 Integrated Sensors... No
AMD K8 thermal sensors... No
AMD Family 10h thermal sensors... No
AMD Family 11h thermal sensors... No
Intel Core family thermal sensor... No
Intel Atom thermal sensor... No
Intel AMB FB-DIMM thermal sensor... No
VIA C7 thermal sensor... No
VIA Nano thermal sensor... No

Some Super I/O chips contain embedded sensors. We have to write to
standard I/O ports to probe them. This is usually safe.
Do you want to scan for Super I/O sensors? (YES/no):
Probing for Super-I/O at 0x2e/0x2f
Trying family `National Semiconductor'... No
Trying family `SMSC'... No
Trying family `VIA/Winbond/Nuvoton/Fintek'... No
Trying family `ITE'... No
Probing for Super-I/O at 0x4e/0x4f
Trying family `National Semiconductor'... No
Trying family `SMSC'... No
Trying family `VIA/Winbond/Nuvoton/Fintek'... No
Trying family `ITE'... No

Some systems (mainly servers) implement IPMI, a set of common interfaces
through which system health data may be retrieved, amongst other things.
We first try to get the information from SMBIOS. If we don't find it
there, we have to read from arbitrary I/O ports to probe for such
interfaces. This is normally safe. Do you want to scan for IPMI
interfaces? (YES/no):
Probing for `IPMI BMC KCS' at 0xca0... No
Probing for `IPMI BMC SMIC' at 0xca8... No

Some hardware monitoring chips are accessible through the ISA I/O ports.
We have to write to arbitrary I/O ports to probe them. This is usually
safe though. Yes, you do have ISA I/O ports even if you do not have any
ISA slots! Do you want to scan the ISA I/O ports? (YES/no):
Probing for `National Semiconductor LM78' at 0x290... No
Probing for `National Semiconductor LM79' at 0x290... No
Probing for `Winbond W83781D' at 0x290... No
Probing for `Winbond W83782D' at 0x290... No

Lastly, we can probe the I2C/SMBus adapters for connected hardware
monitoring devices. This is the most risky part, and while it works
reasonably well on most systems, it has been reported to cause trouble
on some systems.
Do you want to probe the I2C/SMBus adapters now? (YES/no):
Using driver `i2c-i801' for device 0000:00:1f.3: Intel 82801DB ICH4
Module i2c-dev loaded successfully.

Next adapter: intel drm CRTDDC_A (i2c-0)
Do you want to scan it? (YES/no/selectively):
Client found at address 0x50
Probing for `Analog Devices ADM1033'... No
Probing for `Analog Devices ADM1034'... No
Probing for `SPD EEPROM'... No
Probing for `EDID EEPROM'... Yes
(confidence 8, not a hardware monitoring chip)

Next adapter: intel drm LVDSDDC_C (i2c-1)
Do you want to scan it? (YES/no/selectively):

Sorry, no sensors were detected.
Either your system has no sensors, or they are not supported, or
they are connected to an I2C or SMBus adapter that is not
supported. If you find out what chips are on your board, check
http://www.lm-sensors.org/wiki/Devices for driver status.
ideafix:~# i2cdetect -l
i2c-0 i2c intel drm CRTDDC_A I2C adapter
i2c-1 i2c intel drm LVDSDDC_C I2C adapter
ideafix:~# i2cdetect "intel drm LVDSDDC_C"
WARNING! This program can confuse your I2C bus, cause data loss and worse!
I will probe file /dev/i2c-1.
I will probe address range 0x03-0x77.
Continue? [Y/n]
0 1 2 3 4 5 6 7 8 9 a b c d e f
00: -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- 76 --


dpkg -l "linux-image*"
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Cfg-files/Unpacked/Failed-cfg/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Nom Versió Descripció
+++-=========================-=========================-================================================== ================
un linux-image <cap> (no hi ha cap descripció disponible)
un linux-image-2.6 <cap> (no hi ha cap descripció disponible)
ii linux-image-2.6-686 2.6.32+23 Linux 2.6 image on modern PCs
rc linux-image-2.6.26-2-686 2.6.26-21 Linux 2.6.26 image on PPro/Celeron/PII/PIII/P4
ii linux-image-2.6.32-trunk- 2.6.32-5 Linux 2.6.32 for modern PCs
ii linux-image-686 2.6.32+23 Linux image on modern PCs
ideafix:~# uname -a
Linux ideafix 2.6.32-trunk-686 #1 SMP Sun Jan 10 06:32:16 UTC 2010 i686 GNU/Linux
ideafix:~# cat /proc/cmdline
BOOT_IMAGE=/boot/vmlinuz-2.6.32-trunk-686 root=UUID=704e402e-1ea2-40cc-a8e1-f0a86fce428d ro video=i915:modeset=1

But it does mostly the same with kernel params:

splash intelfb:1280x800-24,accel,hwcursor,mtrr

And, like the previous report, I have:

Feb 6 21:44:05 ideafix kernel: [ 0.297913] efifb: probing for efifb
Feb 6 21:44:05 ideafix kernel: [ 0.298018] efifb: framebuffer at 0xe8000000, mapped to 0xdff00000, using 4000k, total 4000k
Feb 6 21:44:05 ideafix kernel: [ 0.298026] efifb: mode is 1280x800x32, linelength=5120, pages=1
Feb 6 21:44:05 ideafix kernel: [ 0.298031] efifb: scrolling: redraw
Feb 6 21:44:05 ideafix kernel: [ 0.298037] efifb: Truecolor: size=8:8:8:8, shift=24:16:8:0
Feb 6 21:44:05 ideafix kernel: [ 0.402860] fb0: EFI VGA frame buffer device
Feb 6 21:44:15 ideafix kernel: [ 45.512960] fb: conflicting fb hw usage inteldrmfb vs EFI VGA - removing generic driver

Now the odd things:

I've added
cat << EOF
insmod 915resolution
915resolution 7e 1280 800
EOF

to /etc/grub.d/00_header to set the native resolution of the laptop display.

And the reason I'm reinstalling it is because the hard disk suddenly showed many errors,
and an fsck left it worse. In fact it has had some hardware glitches in the past, so something
may be wrong somewhere in the hardware too. But it worked with lenny, and since other people
seem to have similar problems I thought I'd say it.

My guess is somehow efifb conflicts with either intelfb or inteldrmfb for the laptop display and xorg finds
it unavailable for EDID and thinks it's disconnected. And
somehow the external crt is not claimed by efifb so that xorg can use it. But it doesn't make sense
because I can see grub-pc and the boot console through both the external crt and the laptop display.
The external display shows a blurry display, but I think it's because it can't handle 1280x800.
Xorg displays only on the external crt at 1024x768

I'll attach hwinfo and the xorg log . I don't know what to try.