Various stories have suggested that all of them have smoked pot at one time or another. Of the more humourous rumors, is the "Larry learning to roll a joint" story from the Pop sessions.
I wouldn't get too upset, I don't think any of them smoke it much anymore. I'd be more concerned about the cigarettes.
EDIT: I knew there was an interview with Bono on the matter of pot smoking somewhere... it can be found at
http://www.atu2.com/news/article.src?ID=2082
"Interviewer: Did you have any reaction to David Blunkett's decision, in the U.K., not to charge people for possession of cannabis?
Bono: Sounds smart. I don't know enough about it to comment, but I could never figure out the hierarchy of sins -- or pleasures, I should say. I could never figure out how one was supposed to be worse or better than the other. It's like alcohol or cigarettes. I think smoking weed probably gives you cancer as well, a lot slower. Like all pleasures you have to not abuse them.
Interviewer: Should we go whole hog now and decriminalise the fucking thing?
Bono: I would think so. Again I'm not really informed on all the issues, so I wouldn't want to make any pronouncement. I guess the thorny issue is substances that are addictive and people who are prone to them. It's difficult to make something freely available that makes someone a slave, like cigarettes, but they do. It's hard to know. The devil's revenge on extraordinary things is to make them ordinary by doing them too much. That's what access can do to people. Make the extraordinary commonplace."
There ya go!