The community consensus is that song lyrics are on topic. I agree with this policy 100% - this could be a great "draw" for the site, too, because it seems like there's a dearth of quality information about songs.
Some lyrics - notably the lyrics of Bob Dylan, who recently won the Nobel Prize and whose lyrics have appeared in independent poetry anthologies - have quite a bit of literary merit. A few other artists are notable for their poetic lyrics, as are certain songs. (Hotel California, American Pie, some U2 songs, etc. leap to mind). On the other hand, I think it could damage site credibility if we were analyzing the Backstreet Boys or Brittney Spears lyrics as "serious" literature. (Nothing personal against those musicians, but still...)
Should questions about any kind of song be considered "on topic," or should the song in questions have lyrics that have at least some literary merit? If so, how would we formalize this into a rule that can be broadly and uniformly applied?