I'm sure they'll tell it right when they get back, but this is what I know from being friends with Laura (the one who played who I've known since we were 4) and Eep (the one with the Party Girl sign) and standing right next to them the entire concert. Eep takes that sign everywhere to every concert. A few days ago, they met Bono. He signed the Party Girl sign and they asked if he'd play it. He asked how many chords it was, what key, etc. Then, at the show last night, he saw the sign during Yahweh and came over to us. He leaned over and asked what key it was again. Then he motioned for Edge to wait. He looked like he was going to tell Edge to play it, but then turned back and said "she can play it?" and Laura nodded and Eep was like "yeah, she'll play it". So he pulled Laura up on stage and she played it.
He has seen the sign before on multiple occasions, just never played the song and maybe never put it together until now that it was the same people every time. I don't care how "weird" it was. At least this band pays attention to fans and lets us really get into the show. If you want Yahweh into 40, I'll e-mail you my bootleg of the Saturday show.