They must most definitely sell their ASICs or risk getting sued etc as pointed out above, but it is also clearly in their interest to delay delivery as long as possible if pre-orders are still coming in, because as soon they start delivering, difficulty will skyrocket, forcing them to drop their price so that it continues to seem possible for potential new buyers to make a profit.
I disagree. As soon as they start delivering, people will cancel their orders with the other ASIC vendors and put that money towards orders with BFL. The smartest thing for ANY of the ASIC developers to do is to deliver units as soon as possible.