A nice article about Johnny Cash by The Edge just came out, and in it he tells about the recording of The Wanderer, the sort of weird Ecclesiastes-spawn on Pop (I referenced it here when Cash died.) It just has all the hallmarks of one of those Holy Spirit events.

I think we all thought, 'Aw, Bono's just trying to create some sort of distraction from the fact that he really has no idea what he wants to do on the song, this is just one crazy idea too far.' But whenever something like that happens I've learned to bite my tongue somewhat, because those crazy ideas often turn out to be the ones that come to pass, and indeed this was one of those. We got on the phone pretty much immediately to try and get through to Johnny, and he said he'd love to come down just to say hello, and, y'know, if it was something he could sing on, great, he'd be up for it.
...[Johnny] did two vocals, that was it, we didn't even get into, 'Could you try it this way?' it was literally a case of, 'I can't quite believe what's going on!' There was a little element of giddiness in the room. After he left and we'd said our goodbyes, everyone just looked at each other and went, 'What just happened?!!'

Uh, yeah... I know that feeling exactly.