Hibernate and s2disk do not work on a thinkpad x200s

The problem happens on a Thinkpad X200s with Ubuntu 11.
The symptoms are:
 if I initiate hibernation from the menus in gnome then the screen goes black then after a few seconds it comes back to the gnome-desktop login screen.
If I execute "sudo s2disk", after writing the snapshot/image to the disk, the computer wakes up immediately (without a login screen). This happens with all of the three options for shutdown configuation of s2disk (shutdown, platform and reboot).

In the both cases, the system run with any problem after the "failed" hibernation.

The laptop is a Thinkpad X200s with intel graphic chipset.

thanks.