I just went through this with AA. When I booked our flight a month prior, there were only a few seats available, and they were all for $$$ (most were $68). I almost gave in and purchased 2 seats for us, but had a hard time spending $136 for 2 seats. I called AA several times to ask how they handled this and was told some seats would be made available at the 24 hr check-in mark, unless the local airport took control of all the seats and decided to assign them at the airport. THEN I found out the flight was overbooked but was told on two occasions that we wouldn't be automatically bumped just because we didn't have a seat assignment.
I checked in at the 24 hr mark and kept checking every few hours to see if any seats had opened up. At some point there were ~ 15 seats were made available, but I couldn't 'select' any of them ---- when I called I was told the local airport had taken control of them. When we checked in at the airport 1 3/4 hr before the flight and printed boarding passes, we found we'd been assigned to 2 exit row seats. So, it did work out fine for us, even on an overbooked flight that they were asking for volunteers to be bumped. Good luck!