The A is a PATA drive, the AS is a SATA drive. Both are going to be sold as Barracuda 7200.10 models, the successors to the 7200.9 line. The differences are perpendicular recording, a 16MB cache, and obviously the maximum size.
It doesn't look like storage is going to slow down for a long time to come. Thanks to Akiba PC Hotline for digging up these documents. µ