From what I understand about DX10. This is not 100% offical, as I have not taken much interest in it, but:
DX10 itself is only for Vista. I cannot see there being any DX10 only games for a VERY long time. Even games coming out now will work on a DX7 card, although not as "shiny"
Not too sure about DX9L. From what I have heard it is an update for DX9 on WinXP. I am not actually sure what it is meant to do. Maybe help support some DX10 features?
The major aspect of DX10 is that it handles system resources a lot differently, and cannot be integrated into WinXP without stripping it down and remaking it. Maybe I will do some reading of official press releases and do another post unless someone beats me to it.
Even 2 years from now, most games will probably run in either DX9 or DX10 mode. A lot of modern games run in either DX8 or DX9 mode. Some either have separate .EXE files. I wouldn't worry too much about it. Reason why the new 8800 cards have created so much hype is their power and "unifying shapers", which give game programmers more freedom with graphics. That may actually be the DX10 feature.
Also, Be aware the TheInquirer is NOT a reliable source for solid information. It was them I believe that said "9.L would be an "emulation" of DX 10.0 for XP"
Weather this is true or not, I cannot say.