I'm running TuxOnIce kernel version 2.6.34-r1 with gnome 2.28.2.
I can hibernate using pm-hibernate as root but I do not have a hibernate button in the shutdown dialog box
but it does have suspend button which works.

Here is some info based on https://wiki.ubuntu.com/DebuggingGNOMEPowerManager.

After running:
echo /sbin/tuxoniceui_text > /sys/power/tuxonice/user_interface/program
echo /mnt/data/system/suspend_file > /sys/power/tuxonice/file/target
I can using Hibernate:
$ dbus-send --system --print-reply --dest="org.freedesktop.Hal" /org/freedesktop/Hal/devices/computer org.freedesktop.Hal.Device.SystemPowerManagement.H ibernate

$ dbus-send --session --print-reply --dest="org.freedesktop.PowerManagement" --type=method_call --reply-timeout=6000 /org/freedesktop/PowerManagement org.freedesktop.PowerManagement.CanSuspend
Error org.freedesktop.DBus.Error.ServiceUnknown: The name org.freedesktop.PowerManagement was not provided by any .service files
Does not work on Gentoo ?

$ gconftool-2 -g /apps/gnome-power-manager/general/can_hibernate
true
$ gconftool-2 -g /apps/gnome-power-manager/general/can_suspend
true
$ polkit-auth | grep power-management.suspend
org.freedesktop.hal.power-management.suspend

$ polkit-auth | grep power-management.hibernate
org.freedesktop.hal.power-management.hibernate

$ pm-is-supported --suspend && echo $?
0

$ pm-is-supported --hibernate && echo $?
0

$ hal-device | grep power_management.can_
power_management.can_suspend = true (bool)
power_management.can_suspend_hybrid = true (bool)
power_management.can_hibernate = true (bool)

I have being pulling my hair out over this missing hibernate button for over a day and Google is being useless in this matter.