Comment 20 for bug 36158

>A large number of these problems may revolve around buggy bios that a crashing when they are told to power off. If at all possible check whether there is a newer BIOS for your motherboard (of course flashing your BIOS is done at your own risk etc.)

I diseagree here on this explonation, as I reported before this buggy behaviour goes and come back on different kernel versions, the 2.6.15 and 2.6.16 are at fault too, really feels more like a change badly done in the kernel than a hardware related trouble.
And btw I have the last available firmware and the result is the same.