I had this problem and it was driving me nuts. For months I have been without power off and S3 functionality. I was THIS close to RMAing the board when I finally found a solution.
The problem is some strange BIOS or hardware bug and has not been fixed even as of the f6 beta. It is possible the bug is a hardware bug only on the V1 board.
I know for a fact that it is a BIOS or hardware design flaw and not a driver problem because you don't to boot Windows to demonstrate the problem.
If you cold boot the computer and press END on the bios start screen you enter a bios utility which purpose is to back up or restore the bios from a drive. In that utility there are menu items that let you either immediately reboot or turn off the computer. If you select the "turn off computer" item, the system turns off and then immediately reboots (if you have the problem.) This happens even if you have just performed cold boot from a power-cord removed state.
I tried repeatedly loading the default/failsafe settings, but that never worked. I even tried flashing to F5 from F3, then F6beta from F5, to F1 from F5, and then back to F5.
Finally out of desperation I used the blue CLR CMOS button on the rear of the computer (with the power off!) and this caused everything to start working properly again. (I also cleared the DMI pool, but I now believe this had nothing to do with the fix since I ended up fixing the problem twice and didn't clear the DMI pool the second time.)
Interestingly enough, the problem reappeared after about 30 minutes when I tried to let the computer auto-enter the sleep state. I had already manually shut down the computer successfully several times as well as manually entering the sleep state and restoring several times. Remember I had been unable to do this for months prior.
As an experiment, I saved the BIOS profile and used the clr-cmos button again, and again the problem went away. I then restored the saved profile and the problem was still gone. Whatever causes the problem it is not touched by saving or loading the cmos settings via bios software, but IS fixed by hitting that blue button.
I then upgraded my INF to the latest version and turned off USB selective suspend and disabled wake timers in the power options dialog trying to find a magic bullet fix that would prevent the problem from appearing and have not since (in two days) had any problems. Whether I stumbled across a fix that is keeping the cmos from getting messed up or whether the cause is random and I just haven't hit it again yet I don't know.
Does hitting the clr-cmos button clear more of the cmos than can be done from the bios utility, or is there some other part of the same chip that latches up and needs resetting that simply cannot be reset any other way because of the constant application of battery power? I don't know that either.
I do know that twice in a row I was able to fix the problem by saving my cmos profile, hitting the blue clr-cmos button with the power off, and then restoring the profile.
If using the "shut down" option from bios utility you get to by pressing the END key on boot doesn't cause a reboot for you then this probably won't help, but it has sure been a lifesaver for me. I am very confident this will fix the problem for some people since I had to do it twice and since the second time it was the only action I performed to fix the problem.
