It funny becasue he lay on the ground singing and managed to make it look as if it as all part of the show,he doesnt seem in the least bit embarrassed.
Its not as if he was injured or anything, he just fell over, the show carried on.
When Bono falls off the stage whilst walking backwards during the Elevation tour (think its when playing UTEOTW),he got up an looked a bit shaken but manged to play on regardless!