DirectX 10.1 adds 5 functions, 3 of which are used to access the other 2. Each of those expose a new feature not available on current hardware.

In addition, DX10.1 requires a few features be available on compliant hardware that were optional in DX10.

DX10 and DX10.1 (and presumably 10.2 etc) exist side by side. It's up the the game developer to chose which one (or both) to target. They will likely target both until there is significant market penetration of DX10.1 compliant cards. Are there even a significant number of DX10 only games yet?