My copy of Vista RC1's DXDIAG says its already running DX10 ... obviously backwards compatible with DX9,8,7 etc.Vista is the launch platform for DX10, Vista itself is written against directx9, so support will always be there.
http://forums.ea.com/mboards/thread.jspa?messageID=1132774&sls=2i would love to be able to play bf 2 on vista hahaha