This is a rather simple detail, but I haven't seen anyone make note of it. In Linux, if the graphical user interface bails on you, you can switch to a different virtual terminal to kill it. Prior to 10.3, you couldn't do that. If the WindowServer died on you, you had to either ssh in remotely, or forcibly restart your machine.
With 10.3 and Fast User Switching, this is no longer the case. The SystemUIServer process, which controlls the menu extras and fast user switching, is not dependent on the WindowServer. Therefore, when the WindowServer bails and you get nothing but a beachball, the right side of the menu bar still works, and you can switch users. Then, from a different login, you can kill the runaway WindowServer process (using Terminal or Activity Monitor).
Note that this only helps when the spinning beachball is caused by the WindowServer. If the kernel is bailing on you such that you can't even ssh in to your machine, then switching users is not going to help and is probably not going to work.
As a side hint: if the right side of your menu bar stops working but everything else is running, kill the SystemUIServer and it will come right back.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040225212536588