According to EW, not only has Snow Patrol's Gary Lightbody been cast in an unknown role, but Coldplay's drummer Will Champion will also appear, as someone he shouldn't have too much trouble getting to grips with, a drummer! The site doesn't have any more info on his character, but we have a few guesses.
It was reported a while ago that HBO were looking for musicians to film a large "party scene" for season 3. Now although it wasn't ever mentioned by name, this led fans to believe that we would indeed be seeing a very significant and controversial chapter from George R.R. Martin's third novel known as the "Red Wedding". Could these guys simply be cameoing as members of the wedding band? Possibly, but the fact that there's a bit of hush hush about Lightbody's role may be appearing as someone a bit more significant. The group know as 'The Brotherhood Without Banners' have several musicians in their ranks (such as Tom of Sevenstreams), likewise the far more sinister Bloody Mummurs - and both of these should be popping up in season 3 at some point.
Anyway, the possibilities are many. But if one more famous musician is cast we'll have to assume they are just putting an all-star band together on the side.