XFX Does that now too. eVGA also has a step up program which is really nice as well.
From my understanding, When a new card come's out. IE: 8900. You place your name on the list an when your next in line, You get the new card, an only pay the difference. Such as if I had a 8800 an went to the 8900, With the 8800 being 550$ an the 8900 being 600, I'd pay 50$ + the shipping.
That's what my understanding of it is, I'm not 100% shure how it work's yet.
An my 8800's was 650$ each. If anything they should be paying me money back after the 8900 is released. LOL If it is 600$.