They've made all five tracks (or "stems") of the song "Nude" available in iTunes for $0.99 each [iTunes link]. The tracks are in DRM-free iTunes Plus format, so you can pretty much manipulate them however you'd like.
Purchase all five before the end of this week and receive an access code for the full GarageBand file. Once you've finished your remix, you can upload it here. It definitely sounds like fun, especially for Radiohead fans (like me).