Here's an ugly fix for getting out of the boot loop:
But the permanent fix is as follows:
Remove or rename /usr/lib/blacksn0w.dylib
Remove or rename /System/Library/LaunchDaemons/com.apple.CommCenter.plist
Phone will remain activated, but will now be locked again
In Cydia, add http://cydia.pushfix.info/.
Then search for and install BlackSn0w RC2 Final.
The side effect of this fix is that the unlock takes place after the phone boots completely (to avoid the race condition). But that means it will take an extra 30 seconds or so for the phone to establish a cellular connection.
I wonder if this fixes various GPS-loss problems as well...?