Last year, we were told that Nirvana's Nevermind would be turning up as a full-album download for Rock Band. Least, we thought we'd been told. Turns out we weren't told anything at all, and that any and all comments Harmonix made regarding Nevermind were merely examples of what could be turned into DLC, not a guarantee of actual DLC. So don't expect it anytime soon. Fine by us - we'd rather Bleach anyways.
Nevermind Never Officially Confirmed for Rock Band DLC [Co-Optimus, via 1UP]