Yes, I agree that smoking, drinking, and poor technique were partly to blame for Bono's voice probles in the past, but....
Sometimes I wonder if Bono's voice problems in the mid to late 1990's were caused by under-use, as opposed to misuse. Think about it: U2 only toured once between 1993 and 2001. Just like anything else, the voice will wither if it is not used regularly. Accordingly, Bono's voice declined as it was used less frequently. Plus, Bono started using a different, lower voice circa Achtung Baby to better compliment the Euro-dance direction of the group's music. When he did go for the high notes, he tended to use falsetto (as opposed to his full voice, as he did in the 80's).
Bono abandoned his lower voice style for ATYCLB, and started going for the high notes with his full voice, just as he did in the late 80's. His voice sounded creaky and rapy on the album, but go MUCH better as the tour progressed. Now he's got his voice back, and it sounds even better on the Vertigo tour. Admittedly, it is not quite as powerful as it used to be, but it is powerful nonetheless. Plus, he is hitting all of the high notes, just like he used to.