I'm game! I beta-tested the Steam version of Spelunky. Andy Hull sent out Steam keys tied to a promotional version of the game, but I believe regular keys should work for testing if they are redeemed prior to the release date.
Achievements are a little tricky to test as only the latest to be unlocked are visible via the Steam overlay, as clicking on them won't lead anywhere until the game is released. So keep this in mind, testers! You could press shift+tab after fulfilling achievements to confirm it's functioning.
I can test the game on Windows 7 and Vista, both 64-bit.