They'll bump business class if you volunteer. I'm pretty sure you're well-protected from an involuntary bump when up front.

I played UA one night when they were grossly overbooked at the end of a holiday weekend. Got the operational upgrade, then immediately asked to be bumped as long as I could keep the FC seat on the Monday morning flight. Ended up with the typical basket of goodies: hotel night, meals, FC in the morning, $400.