During push back; an A319 Captain is asked to set brakes. When advised by the Captain that the aircraft is still moving; the tug driver explains the tow bar has separated from the aircraft. No damage occurs.

Narrative

Pre-departure was routine; though we were cleared to push about 3 minutes late due to late passenger arriving and getting settled. Received clearance to push from Ramp after an aircraft cleared behind us. I relayed to ramp with standard terminology. He told me to release brakes; which I did. We sat and waited for the aircraft to move. Shortly thereafter; we started to push back; and about 10 seconds later I got a call in the headset to set brakes. I mentioned that we were moving; and the push crew said he knew; the tow bar was disconnected. I applied the brakes and when we stopped set the parking brake. When the ramper came back on the radio; I asked what happened and he said he hadn't locked the tow bar and it had disconnected. I estimate we had rolled about 8 FT. I asked him if he and everyone else was OK and he said he was fine; but that this is what happens when people are rushing you to depart on time. I agreed and told him that I would write up a maintenance action so that we could get the airplane inspected for damage.