I know there have been a lot of hints/hacks/tips/etc for trying to get rid of the top Apple menu bar, but most of them seem to be neither universal nor easy to implement.
In browsing Apple's Developer Connection for information about crash logs, I happened to come across their Guide to Creating Kiosks on Mac OS X. Interestingly, there's a demo application called UsingSystemUIMode (56KB download) which, among other things, allows you to hide the Apple menu bar with one click!
[robg adds: There are some other interesting things in the documentation, as well as the sample app -- information on how to permanently hide the Dock and disable the Apple menu, for instance. Note that these are programmatic solutions, not defaults write preferences tweak. I assume, but don't know how, that it should be possible to write these preference settings into any app where you'd like the menubar hidden, for instance. Any takers?]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060329115234642