I'm pretty sure they don't drink -- at least Bono, for that matter. I'm quite certin that whatever is in that bottle is non-alcoholic. I'm pretty sure there was an interview with Bono in Continental magazine back in May/June which detailed his allergy to alcohol -- he mentioned how he fell asleep under a table at dinner once, and how he fell asleep at mixing desk on the floor of a Sonic Youth (LOUD) concert a few years ago. Since then, he has been disgnosed with an allergy to alcohol, and now he doesn't drink, except for the occasional and seldom glass of fine wine.
The liquid onstage is akin to orange juice,l from what I've heard -- I head it's orange and even thicker than OJ. It's probably a high-glucose drink to (1) give him energy for the show, and (2) to coat his throat so it won't become hoarse during the show -- like cough syrup (Vicks Formula 44 vs. Robatussin), the thicker it is, the better it soothes your throat.
As further evidence of this, remember that the band did not play in Las Vegas on the first leg of the tour. Why? He and many other artists are quoted as saying that the super-low humidity in Las Vegas wreaks havoc on a singer's vocal chords/throat (Remember the opening show of the PopMart tour?). Bono was quoted as saying that he would never paly in Las Vegas again, unless an the stage were filled with humidifiers (I'd be very interested to find out whether the stage at the Thomas & Mack center in LV has a bunch of humifiers on it). So, they'll be playing LV at the end of their tour -- as late as possible, so it will be as cold as possible, so that the relative humidity will be as high as possible. That's my take on it.
In sum, from all I've heard, know, and understand, Bono doesn't drink