I'm sure I read XP is the primary OS for Steam users somewhere in the last month or two. If this is factual then it's truly absurd. Fair enough game companies don't want to bother with anything earlier than the prior OS, in this case XP, Win95, Windows 3.1, DOS etc but XP's still so mainstream it has to be aimed for. Maybe when XP users migrate to 7, Vista and XP can be dropped from the system requirements but not yet.
Just checked Steam Survey page, it appears that most popular system on Steam is Win7 x64 XP is on second place, though.