Basically, if the API is not changing a lot, providing optional support for 10.1 is trivial, just a few lines of code in how you initialise the DX objects. The larger issues of whether to go with DX10 or not are related to market share - and interestingly the rate of OS upgrade seems to be the limiting factor. There are about 4x as many gamers with a DX10 graphics card and XP out there as there are with a DX10 card and Vista, and although even with the XP users in there right now we're still only talking about 10% of the player base overall.

http://www.theinquirer.net/default.aspx?article=41643

No surprise there.