Wow... lets just clear this up for you guys... canadiens1160 is the only other person who has any idea what is going on here...
Rather than a few guitar parts here, a few keyboards there, a few vocal harmonys here...etc... the reality is that U2 plays about half of their set to a click track (like a metronome). This means that there is a backing track going throughout the WHOLE song. However, it is not often very audible except for in certain parts.
The most backing track-heavy song of this tour is probably Miracle Drug. You will notice that if you listen to the San Diego bootlegs the band is having a hell of a time staying in time with the backing track. Like canadiens1160 said, as the band reaches different parts of the song, the techs will start new programs so that they can try to stay in time as close as possible. So, for Miracle Drug, I would assume there are 5 or 6 different backing tracks that start up at different times during the song. HOWEVER, U2 may just use one backing track which plays for the whole song, in time with the click... this is why the band needs to stay in time.
I assume that Larry, Adam and The Edge all have the 'click' playing in their earpiece at the start of any song that uses a backing track. After the song has started, The Edge's click, and maybe Adam's as well, goes away or turns down. Larry is always playing to a click for songs with backing tracks. Im pretty sure this is why The Edge isn't all over the place as much as he would maybe like to be... with all his different effects and backing tracks and what not, he needs to keep shit organized...
I think it is fair to say that Bono does not listen to a click in his earpiece...
As for some more specific stuff:
Beautiful Day has plenty of vocal backing tracks, but the Edge is singing as well...
There are very few Bono backing tracks... during the Elevation tour I think he had them for the OOOs in elevation and there was some pre recorded bonos in the backgroud for beautiful day, no words though, just aahhhs and stuff... I haven't noticed any bono backing tracks so far this tour...
I hope this clears up some stuff for you guys... if you have any more technical questions I can probably answer them... I have played in bands with and without backing tracks...