Ooooh tough one!
Geldof's standing orders for the original Live Aid was for an artist/band not to play the most recent record, because it would be a cheap shot. Adam Ant got a bollocking for singing "Vive Le Rock", as opposed to hte more well known stuff form the early 80's.
Assuming it's the same rules as then, I don't expect to hear COBL (shame - I love it). It's probably reasonable to assume that as not everyone there will be a U2 die-hard, they will probably play the most recognisable U2 "anthems" if you want to call them that.
I reckon:
Beautiful Day (In reference to the day itself)
Streets (with the "Africa Africa" rap thru it)
One (It just suits the overall spirit of the day)
Hopefully they will actually get at least three songs out of the day!