I just discovered something a little odd but useful. If you have a Logitech USB headset and you use Skype, make sure that Skype is not open before plugging in the Logitech USB headset -- otherwise, you get a full-scale system freeze that doesn't respond to anything but the power button.
Plugging in the headset and then opening Skype, however, does the trick nicely.
[robg adds: I can't confirm this one, but if someone can (or cannot!), please post in the comments. I'll then update the hint as needed.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060710105247296