The kernel crashes happened regularly only with 10.5.1, when I inserted my Samsung mp3 player. The player works perfectly with 10.4 and on all other systems. I reported the bug to Apple, they said it’s a ‘know issue’. Later upgrades fixed the crash when the reader was inserted, but the reader is still not usable with 10.5. I had only a few kernel crashes after that. Mail keeps crashing often, though.