You have to have a game thats designed in DX10 otherwise any game is running DX9c even though your running Vista with a DX10 game. Games that play in DX10 are few so far I believe we have Lost Planet, Dirt, Bioshock on the 21st, not sure if shadowrun is DX10, Halo2, Flight simulator X may have missed some but those are the ones I know of.
If your playing something like BF2142 CSS etc your just reverting back to DX9.0c
DX10.1 is just upcomming version of DX10 that allows shader model 4.1 etc. It appears from the post that 8 series will support it with original estimates that only the 9 series cards will. DX10.1 is in a beta form currently as sp1 for Vista however its not available to general public yet and although it contains drivers for 10.1 although since we don't have a public driver or firmware update it will still show as DX10.
If you put a 7 series card and install vista you'll see in dxdiag it would just show DX9.
celegorm can you also confirm the new line as the 9800gtx or was my original post inaccurate and it is in fact the 8900GTX?