Not only did AMD cut the memory interface in half for the Radeon HD 2900 GT, but they've also reduced the amount of stream processors available.

The previous two cards in the Radeon HD 2900 family have 320 stream processors, but the 2900 GT will have to make do with 240.

This could have a more serious impact on performance than the 256-bit memory interface, although time will tell how much of an impact this will have.

With the upcoming RV670 it seems like AMD has just rushed out the 2900 GT to have a product to plug a hole in it's lineup until the new cards arrive. 

The 2900 GT isn't expected until mid October, which might indicate that the RV670 isn't as close as we expected, or it doesn't perform as well as AMD would've liked it to.