If U2 is Christian, more power to them.
If one cannot tell from their music if they are or not, then I'd say that THEY are the ones going out of their way to prevent themselves from being labeled as Christian.
In which case, I'd go with their own desires and say that they are not a Christian group.
I have no problem with rock, rap, soul, etc. being having Christian lyrics and coming from Christian singers. I just think that if you're gonna be one, then don't be coy about it.
Coy to me means that you've not made up your mind.
Oh, they're anything but coy. They wear their faith - struggles and all - on their sleeve. I think they're not labelled as Christian, however, because they don't fit into the Christian music cliche.