I am unable to suspend in a fully updated (as of yesterday, 29 March 2011) Ubuntu 10.10 installation (kernel 2.6.35-28). Following is a list of some of my hardware:

Motherboard: Gigabyte GA-X58A-UD3R

Video: Radeon HD-567X-YNF3

Initially, when I went to either Suspend or Hibernate the machine would almost go into suspend, but it would never power down. Instead it would bounce back to the login screen. This was due to a problem with the USB3 ports being unable to suspend (noticed this in /var/log/kern.log). Disabling the USB3 ports in the BIOS fixed this issue.

Now Suspend and Hibernate power down the machine. It successfully awakens from Hibernate. However, it will not return from suspend. The mouse and keyboard are not powered and the monitor has no signal. These devices are still not powered after a restart. I must power-cycle the machine. The pm-suspend log ends after it states that it entered suspend (i.e. there is no information any resume code running).

I discovered that acpitool -s suspends the machine and resumes successfully exactly once. The second time the machine will not resume. I am not sure how these two tools handle suspend differently.

UPDATE: the problem was introduced somewhere between 2.6.35-22 and 2.6.35-28. I have both kernels install presently. Suspend works fine with 2.6.35-22 but not with 2.6.38-28.