My phone just rebooted in the middle of a call. The phone was charging via USB at the time.
Actually, I'm not sure it really rebooted. It may have hard crashed into the Apple logo. I had to force a full reboot.
It's never done that before.
I suggest waiting a while before updating to 2.2. If you do update, do a reboot immediately after the install.
Update 11/22/08: The phone hasn't spontaneously rebooted since the first event. The MPR App died with the OS update -- crashed on selecting a "channel". I deleted the app on the phone, checked for updates (none, I'd already updated it once recently), and reinstalled from iTunes. that fixed the problem.
Update 11/30/08: No further problems, updates working, no more intra-call reboots.