This is weird...
I just had my second ever random reboot with CM7. I unlocked the phone, and tried to switch homescreens, and the next thing I see is the CM logo. It sipped the Moto logo, and went straight to the CM logo. What's even weirder, is that my uptime widget didn't recognize it as a reboot. It still says over 1 day, 20 hours of uptime.
The first time this happened, I only felt the little vibrate that happens just before the boot. Then I pulled the phone out of my pocket, unlocked it and noticed the same thing. It had just rebooted (all of my widgets were loading, etc.) but my uptime widget was unaffected. I thought I was imagining things, but this time I actually WATCHED it reboot.
I'm currently on Build 38, no idea what build I was on last time it happened.