I think this is part of Nvidia strategy, to release as many cards as possible just to make ATI products "invisible" for the usual buyer. When you see a shelf full of nvidia cards ( 9800 GTX, 9800 GT, 8800 GTS512, GTS 640, 8800 GTX, GSO, bla bla bla) and just one or two ATI cards, guess what you're gonna buy!? One of those GT something cards. The whole internet is full of reviews about Nvidia cards, because there is one Nvidia card released per month, so all the hardware sites have to review it and this is how the regular buyer starts to forget that there is another performance videocard manufacturer on this planet.
I don't see any other reason for Nvidia to release the 9800 GT ( die shrink :Q ) , 8800 GS when there is a 9600 GT, or rename the already mentioned GS into 9600 GSO. Also, the 9800 GTX is again pointless when you have the 8800 GTS, which has about the same performance for far less money.