If you are running from CD and have no devices mounted, you can press Alt-F2 and enter
Code:
sudo poweroff -f
and the computer powers down within just a second.

IMHO, this is fully sufficient if running from CD.

Coming from Mac OS 9, I am not used to waiting when actually trying to shut down the computer. (The bootup time is also a problem. Hopefully Kernel 2.6 will allow for reliable Suspend-to-Disk...)

(Does anyone of the experts know whether the HDs get "synced" when using "poweroff -f"? If yes, one could possibly even use it with HD installs - but I don't recommend it.)