Some songs do deliberately have backwards messages in them, but when you hear them, they are obvious. There is an example on Pink Floyd's "The Wall", on an ELO album, and others. These are snippets of actual reversed tape. They sound like unintelligible backwards speech when you listen to them.

The idea that a normal lyric, when played backwards, will say something different, is simply a case of people hearing things they want to hear in random groups of syllables. For instance, there is the idea that you can hear backwards messages in "Stairway to Heaven". I've played with this myself, and when you hear the results, you can tell that any backwards messages were purely accidental. Yes, there's one bit where it seems like Plant says "here's to my sweet satan" (by reversing the line "there's still time to change the road you're on"), but it's totally random and accidental.

More details at this site:
http://fly.hiwaay.net/~crispen/rants/secrets.html