From kevchino news:
To make remixing easy, the separate 'stems'* from the song are available to purchase from iTunes here. The 'stems' available are bass, voice, guitar, strings/fx and drums. You can mix them in any way you like, either by adding your own beats and instrumentation, or just remixing the original parts.
If you purchase all five 'stems' from iTunes during the first week they're available, you'll be sent an access code to a GarageBand file ready to open in GarageBand or Logic. However, you don't need GarageBand to do a remix, all the stems are in iTunes Plus format and compatible with several music software platforms.
Finished mixes can be uploaded here where the public will listen and vote for their favourite remix (voting ends May 1st). You can also create a widget allowing votes from your own website, Facebook or MySpace page to be counted as 'mix votes' back on radioheadremix.com. Radiohead will listen to the best remixes.