AS for PCI Express shares a common heritage with PCI Express Base, as it is known today. AS and Base use the same Physical Layer but differ at the Data Link and the Transaction Layer. Don't expect a PCI Express device to turn into an AS device because these layers are typically implemented in hardware. AS is still a few years away from regular deployment, though some proof-of-concept systems will be built this year.
Unlike PCI Express Base, AS is designed from the ground up with a message-based, peer-to-peer switch-fabric architecture. It has backwards-compatible support, allowing a PCI Express Base subsystem to be bolted into an AS switch fabric. However, this is done more to take advantage of the anticipated rush of PCI Express-Based hardware.
AS faces major hurdles, as do most other backplane switch fabrics, such as InfiniBand. New application programming interfaces (API) must be employed to take advantage of the message-based architecture. Also, applications need to be written or modified to do the same.
AS will begin to play a role in about a year as hardware starts to arrive. Meanwhile, it's PCI Express all the way.