I've probably got the facts wrong, but as I understand it.. at one point (around the time of Beaucoup Fish, or probably earlier), U2 and Underworld wanted to work together to create a collaborative track. It never saw the light of day because (to paraphrase Rick) "it sounded like an Underworld remix of a U2 track, rather than a track by Underworld and U2". U2 were apparently quite happy with it though.
I think the interview where they talk about this is the same one that features the 'Born Slippy Unplugged' song.. so keep an eye on
that thread if the mp3 shows up.