Here's a picture of our ball possessor:
http://www.chiefdelphi.com/media/photos/35345?
Basically, on either side is a set of wheels that spin inward towards the robot. As we contact a ball the wheels attempt to drive the ball inward towards the robot until they physically cannot, at which point the ball is pinched into the mechanism. At that point the ball is pinched in the mechanism and can't escape unless the mechanism is powered in reverse or the kicker is fired. We used a window motor for this so that we wouldn't have to stall the motor to keep the ball pinched in place, but this could be replicated with any motor as long as you come up with some system to prevent the motor from burning out.
There are two down falls to this sort of mechanism though, the first of them being the opening size of the pincher. The effective opening is similar to that of a suction cup so it requires a certain degree of precision to grab a ball. We found that as long as the center line of the ball is within ~4" of the center line of the robot we're fine. The second down fall is that I mounted the possessor rigidly to the frame of our robot. This can and did lead to at least one carrying penalty because the ball can leave the floor as we drive into the Goal, and also as we encounter bumps in the playing field. We plan to revise the design for the Championship to make it both more robust and "Carry-Proof"