Had my first hard reboot crash on Beta 3 this morning. I enabled tethering, did a few things on my laptop, and then tweeted with Twicca on the phone. Performance slowed to a crawl, Trebuchet force closed, and then the phone rebooted itself.
I did a full wipe before installing.
EDIT: Hard reboot crash #2. Again while tethering is active. I switched back and forth between Gmail and another application. Things got really slow, and then the phone rebooted itself. I ran the Fixed Permissions utility and rebooted. Let's see if that helps.