If you read the various leaked specs of nvidia's new gpus you'll see why.

I am wandering the same thing.

The word from inside the biz is that GK104 beats GTX580 and 7950 and gets close to 7970. At the least somewhere inbetween. This is where he gets that from.

7850 and 7870 look like great parts though, 7850 is just shy of a GTX570 overall, and 7870 is on par with a GTX580.

We will wait and see what Nvidia come up with but don't be surprised if the GTX660 beats even the 7950 as well as these cards. After that it depends on pricing and availability. If nothing else the Nvidia cards will hopefully reduce prices somewhat.