After upgrading two machines to the latest 2.6.26 kernel from sid, they exhibit extreme time drift (to the order of 10 seconds in a minute), resulting in clocks that are hours out-of-sync after only a day of uptime!
The drift is so bad that ntpd gives up trying to sync the clocks and rejects all sources (it is the same basically with or without ntpd).
The affected systems ran perfectly with the latest 2.6.25 (debian) kernel and the behaviour started only after rebooting with 2.6.26.
These are not production systems, however database activity is severely affected, and so are many other programs that depend on time like VPN software which refuses to establish connections.
I tried manually installing an older 2.6.25 kernel on one of them and the problem went away, so it is definitely something in the latest kernels (didn't try previous 2.6.26 versions).