Well I heard that they're a kind of security blanket for Bono, as a barrier between himself and the world. Apparently the whole celebrity thingy started to overwhelm him, and the glasses came between him and that celebrity thing- watch the clip for HmeTmeKmeKme- there's a bit where there's a newpaper with a pic of Bono flying with a halo above his head, and the caption reads something like- "So righteous, so pure, so what?" and then he fashions the halo into glasses, and then the cartoon Bono is jusxtaposed with a real life pic of him in Fly mode. Another scene has people photographing Bono, and the flashes are blinding him, causing him to stumble and appear confused, and then Larry I think throws him some sun glasses, and Bono puts em on and is strong again.
I think the glasses have something to do with the way Bono remade his image from the late 80's JT uber serious look to the fun, glamorous 90s look. Since then the glasses have become his trademark.
Interestingly, the style of glasses that Bono wore on the Elevation tour 3 years ago have come into fashion here in Australia- dunno if you other countries have em too? And now everyone is wearing them, even people who don't know who Bono is!