We've been waiting to play Nirvana's Nevermind in Rock Band since it was (kind of) announced. The seminal grunge album was mentioned as DLC at E3 2007. Sadly, it might never happen. In fact, Harmonix says they never said it. Harmonix said they mentioned Nevermind only as an example of something that could be available.
Thanks for spelling out what a rock and roll album is, Harmonix!