IE8 runs on XP, though. Just about every Windows computer still in operation should have IE8 installed, for security reasons if nothing else.
We still officially support 7 for our offsite/embeddable stuff, but from the chatter I hear of the devs that are working on that stuff, the maintenance cost sounds higher than the revenue. Then again, that cost to us is yet another reason that people like our API, since its one less tedious thing for other devs to worry about in their checkout process.