Yeah system requirements state vista or above, which is odd given that it uses the unreal engine and dx9 - but it doesn't seem to work with XP according to the users on the offical forum.
Steam pre-order rewards are at 88% on tier 3 now, which means they'll easily unlock by launch (1 week, 6 days), so everyone gets a free copy of Civ 5.... although I've already got one so it's not much use to me