Given the fact that neither game required a Direct X 10 videocard, I don't think I'll call this overly surprising. Also, the console versions of the games both run on older versions of Direct X (Halo 2 was on a DX8 based system, Shadowrun on a system that's more than DX9 but not quite DX10). These are not Direct X 10 games - they don't even require Direct X 10 video cards (see the system requirements for Halo 2 and Shadowrun).
I don't *think* Microsoft has gone out and claimed that neither game will run without DX10 (and if they have, can anyone provide a link?). I can appreciate them wanting to help encourage DX10/Vista game development, and they passed up a LOT of money by releasing those two games on Vista only. But to be honest? They would've been a lot better off supporting Vista and XP, showing gamemakers how to make games that work well under both, helping provide a smoother transition between the two OS's and DX's. That would be more beneficial, I think.
Now if only ATI and Nvidia would get their act together concerning drivers :P