I don't know what Bono's prognosis is, but I do know he was very mobile at the 2011 gig I attended. He seemed less mobile at the 2009 Chicago night 2 gig, though (I'm sure it was probably just for show, but he even laid on stage and started a song that way). Based on what I've heard, the surgery was very successful and he recovered very nicely. Of course, that could just be PR spin and it's possible it's changed since then, but given what I saw of his 2011 performances, I'd say his back is in great shape.
That having been said, it doesn't appear that he's working out much. Which honestly surprises me, because being the celebrity he is, he should be able to get a great personal trainer to help him with the right workouts to strengthen his back and slim down a bit. Who knows, maybe he does have one, but it doesn't look like it. From what I've heard (and based on what a friend told me who also had major back surgery) it's absolutely crucial to stay in shape after the surgery. Weight gain can really, really hurt the back. I'd hate to see that happen here.
As has already been mentioned, Bono isn't a guitarist. He's said this a lot and it's true. I don't think he'll change and suddenly become a good guitarist. I also don't see him just sticking to one area and playing his guitar--I see him moving around and working the crowd just as he always has. Of course, I think it'd be great if he could play rhythm on a few songs to sort of beef up the sound (it works well for many bands to have 2-3 guitarists), but only if he's able to do that and sing well.