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?]

