If you normally leave the dock visible, it sometimes gets in the way when apps (especially Classic apps) take over the whole screen. It's a pain to mouse to the dock, switch the setting to autohide, do what you want, and switch it back to always show.
A much easier method is to hit Command-Option-D, and the dock will hide. Do what you need to do, and hit Command-Option-D again, and the dock will show itself.
Caught this one on the MacFixIt boards tonight...of course, from comment #1, I should have 'caught' it by just looking at the Apple menu! Oh well, it was a long day!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010402014419948