I downgraded to Arduino 1.0 and was having better luck, but then today I've had a couple immediate reboots using 1.0 and a Diecimila board. It seems to have the most problems when the Serial Monitor window is open and I either try to upload to the board or try to close the Serial Monitor window.
The OS X kernel panic crash report didn't have anything obvious to say.
I'm going back to Arduino 1.0.2 and continuing the investigation. I'm thinking maybe it's something in the latest OS X Mountain Lion update (I'm running 10.8.2).
- Chris