Nvidia said in some of its soon to be announced documents that DirectX 10.1 won't be supported on the Geforce GTX 280 and Geforce GTX 260 cards. The interesting part is the reason why.
Nvidia claims that "DirectX 10.1 includes incremental feature additions beyond DirectX 10, some of which Geforce 8/9/200 GPUs already support (multisample readback for example)."
The green GPU company said that at the initial design phase of the GT 200 it talked to key developers and their feedback indicated that "DirectX 10.1 is not important" and therefore Nvidia decided to focus on delivering better performance and architectural efficiency.