Here's a mad question, why would you (or anyone) want Vista? What does it do that XP can't?
DX10. Supposedly MS won't release it for older operating systems including XP.
So it kind of depends how 'the industry' will respond: If they switch to DX10 en-masse then people kind of 'have to' buy vista to be able to play any games... OR more realistically, DX10 will be slow on the uptake as well especially *because* there are still so many people running something other than vista.