Yeah, that's what I was getting at. Something like that is pretty much immune to naive brute force, even if we count "Smells like teen spirit" as a word. My guess would be that if it does get cracked, it would be by searching [lyric]+", and"+[some kind of Markov attack], but I honestly have no idea how one would work out the entropy in that model. It depends a lot on how the search is carried out, I think.

I guess we'll find out when passphrases become common :)