Obviously, his voice is sensitive and tires easily. It can very well be that during the tour, he will have his bad days again, vocally. I don't know if or how he can work at that and what could possibly help him, apart from NOT singing so many concerts in a row. Bono has a very good voice right now, the quality of his voice says nothing about how much stress his voice can handle on a longer tour. Other singers might have less problems with that, but Bono clearly has the disadvantage of having a voice that is easily affected by several factors. I'm a little scared to think about him over-stressing his voice again. Right now it's great and I'm sure we will hear this voice on the album. The tour is another thing.
The same talk was before Vertigo tour, people thought that his voice only would be strong on the 1st and 2nd leg. The thing was that his voice got stronger for each week, stood still abit on 2nd leg(outdoor is a bit troublesome for singers, allergy, air...), 3rd leg was stronger than both 1st and 2nd leg. He pushed his voice alot on alot on the shows.
4th leg he was a bit tired, but only in his low voice, he hit the high notes easily without problems.
Seven months rest(well not a big rest, they where recording alot here) and the 5th leg began, and his voice was much stronger then any of the other legs from the same tour.
Also he hasnt rest much under 2007-2008, alot of studio work and speeches and his voice has contunie to devolope and get stronger.
And was suprise me most is that on every ONE NIGHT performence(tv, events...) he has done before 2007 has always been a bit shaky/weak.
But Mandela, Union and the last one his voice is amazing strong.
The key to all this might be that never stopped sing, it is just like if you stop exercise your muscles will be smaller...
He also have done alot of training and also use alot of song techniques.