Comment 65 for bug 107093

Svaens, instead of holding down the power button when your system freezes, try this alternative: Alt+SysRq+"R-E-I-S-U-B".

What you do is you hold down the Alt and SysRq (PrtSc) keys, then type "reisub", pausing a few seconds after each key (since each key does one thing, e.g. 'b' will reboot the system).
Ref: http://lifehacker.com/software/linux-tip/gently-restart-a-frozen-system-298891.php
For more info: http://en.wikipedia.org/wiki/Magic_SysRq_key