The most famous U2 tattoo fan is "Tattoo Dan", he has "One" in several languages on his back, a Joshua Tree and some more. His whole body is covered in U2 tattoos.
I once had a photo of his tattoos but can't find it so soon. He was also on stage with Bono during the 3rd leg of the VT.
[update] Well, I found it, check it out, it's insane:
naaaaaah! thats way too much. these guys would want to get a grip. if you were in a band and someone came up to you with tatoos of you on their body you'd think they are fuckin nuts. and you'd be right. its all a bit too obsessive i think.
when noel gallagher was on soccer AM a few months back a guy sent in a pic of his back tatooed with all the lyrics of the Masterplan. It went from his neck right down to his waist. Noel Gallager : 'i'm not so sure my lyrics are allowed to be reproduced in that way, there could be a few quid in that for me'. how gutted must that guy with the tatoo be lol