OS X hint in Macworld on how to use Spotlight to search for lyrics in iTunes. Added to my ‘much-ado-about-not-much’ file.
There are two fairly large caveats: first, Spotlight will only search MP3s, not AACs or any other file format, and second, you must have previously added the lyrics to the songs.
Or, to grab and add lyrics to iTunes: CantoPod, DeskTopLyrics, and, my favorite, Harmonic.