During the original xbox, you may remember that Microsoft went with nVidia for the graphics chip in the system. Long story short, disagreements arose regarding the monetary agreements between the two companies, and MS ultimately took nVidia to court (though it was later settled).
During that time, MS was also working on their DX9 API for Windows, and speculation suggests that MS sort of left nVidia out in the dark regarding the specifications and such (out of spite, perhaps), which resulted in the infamous "FX Fiasco" lineup of graphics chips, that were quite inferior to that of their competitor's series (ATI, the designer of the current X360 graphics chip, Xenos).
Mind you, much of nVidia's downfall with the FX series was a result from poor yields / bad process tech, but many analysts suspect that the sour dispute between MS and nVidia had a role to play in it all.
Today however, I don't believe that's the situation (atleast in the graphics division of things). nVidia is really the only supplier right now of viable DX10 solution GPUs, and MS is trying to push Vista like no other.
That might not answer your question, but I thought I would atleast mention it, nonetheless.
