We know that the 7900 GS has to end up slower than the currently public and shipping 7900 GTX. This mobile chip is actually meant for the big desktop replacement notebooks - seventeen or nineteen inch notebooks that will offer a lot of power and barely any battery life.
We know it's a PCIe interface card, probably packed in MXM III packaging 80x100 MM, it uses 256 bit memory interface and by the specs it looks slower than the 7800 GTX GO.
You can check more at this Nvidia page µ