Sometimes his smirk is sooo sutle that you don't even notice it! Like here when he was signing my book (you can just see the top of my red head, the shorter one!) too busy looking into his eyes!!
The smirk, to me, is an infuriating expression that says the smartdonkey thinks better of himself than others, but on Bono it seems to be a vunerable self-consciousness of self-deprecation and a fun-loving devil. Who can resist it? Or him?