First the lyrics have to be in the ID3 tags, and I don't think they usually are for songs unless you add them. I use my iPod mostly for podcasts, and sometimes people will put the show notes in the Lyrics field. If they're there, you can get to them by tapping on the screen below the volume bar (on an iTouch in portrait mode) or by clicking the center button on a click-wheel iPod a few times.