I cant agree with that, since that would be the stupidest thing the game developers could do. did you know for example that the first 4 days a game is released, will show if a game is a succes or not, keeping that in mind, how many people would buy a DX10 grafic's card (with most likely a new pc) just to play a few new released games?
I think if they wernt backwards compartible, the games just wont sell good enough for the developers to invest time/money in it. hence, i think DX10 games will run on dx9.0c cards, but i gues they will be emulated or so.
either way, untill DX10 is the standard, games will run on the older cards too, and DX10 is far from beeing the standard yet.