Damn......how hard is it to not fall off of a stage??......I mean we're not talking about a balance beam or anything here.
He tripped over a wire and stumbled. Probably not taped down well or something. Initially they thought he was going to have to go straight to the hospital (and he probably should have). He felt bad that they were only a few songs in, so Taylor (the drummer, who has an 80's metal cover band called "Chevy Metal") took they lead and they played a few songs to at least give the crowd something. After they took Grohl back stage and got his leg wrapped up, he asked if he could finish the show. The paramedic (pictured) said "uh, no...I can't even take my hand off your leg". Dave said something to the effect of "well, then you're coming on stage with me 'cause I'm not leaving".
he tells the story here, starting around 6:45. (correction, it was more like 10')