I think it is integrated in one of "multi-functional" chips, it's not a discrete chip (or maybe it is a discrete chip)?. Broadcom comes to mind from one of the teardowns someone did. Not really sure on the maker. Check ifixit.com, I think they have details at least for the iPad 2.
This is the iPad 2 from iFixit: It also includes a Broadcom BCM4751 Integrated Monolithic GPS Receiver, which is an update from the BCM4750 found in the GSM iPhone 4.
It's quite good I think, without inserting a sim card (I don't know what carrier's aGPS signal it would use), I can get usable signal from back seat of car (without windshield) Haven't tried it in a bus yet.