It seems up to this decade, there were mainstream bands that were definitive music of a decade. For example, the 90s had (as a couple examples):
Insane Clown Posse
However, I don't see many definitive bands of this decade that are popular but:
Of course, there are plenty of other bands (Radiohead was mentioned) but they were in the 90s, and not really this decade phenonomon. When looking at playlists of people around, its usually older stuff, or newer CDs of older bands. This also shows when you hit the Billboard top selling albums. First is Metallica's Death Magnetic (and I can't really call Metallica a band of this decade), and the rest are bands that are pre-millennial.
This is of course not factoring the many, good bands out there that are not in the top pop charts, but just a mainstream snapshot.