I don't know if anyone's pointed this out (a search didn't find it), but if you launch the Printer Setup Utility and click on its application menu, there's a "Reset Printing System..." menu item. Choosing it requires you to enter an administrator password, and it then kills the existing cupsd daemon and launches a new instance. This may help clear up printing problems without a reboot.
[robg adds: Generally, menu items aren't published here as hints. However, given that this one is hiding in the application's menu itself, and the number of times that printing problems seem to crop up, I thought it worth the mention.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050519111021500