I have a similar issue. After updating to the latest version out there (mine is the R2D2 so it takes awhile to get them) it happens less often. It only happens ever few days but seems really random. I also have a problem with the screen being unresponsive. I can navigate through the keyboard if the phone is unlocked but the touch doesn't respond until after a reboot. I wouldn't mind seeing how people are fixing this as it is really annoying and I am sure that Ticklemonster's wife would like an answer too.
Ok, we figured out what the reason is. My wife slided the keyboard out to wake the phone up. When she stopped doing this and started using the button at the top, the phone stopped rebooting. Or at least it stopped rebooting as much as it was before. Who know's what else she is doing.