On the 500 MHz iBook, Keynote has what is now a known problem. It crashes the operating system reproducibly during a presentation under OS X 10.2.3. Apple's Knowledge Base covers the issue (with no real resolution) in Article 61858.
Apparently downgrading to 10.2.2 should solve the problem until the bug is fixed, but I haven't tried that yet.
[Editor's note: Normally we don't consider KBase articles as fodder for hints, but this one is clearly worth noting. I was bitten by it a couple of times myself on our G3/500 iBook (thankfully never while presenting), and it is a bad crash -- the entire OS locks up tight, which means you can't really smoothly relaunch the app and pretend nothing happened!]

