I did it once...camped out for concert tickets on the ground in front of Centennial Hall at the University of Arizona (not U2). I was allowed to leave to attend my US History Reconstruction to Present class, but my two roommates were still in line. If we had all left, we would have been booted.
That was a nightmare night. Hardly any sleep, then when the time came for tickets, the Ticketmaster computers overloaded and they were unable to issue tickets. They had us go home and come back two hours later. In that 2 hour period, the computers had come back up and they didn't hold the line...they just sold to all callers and walkups. When we got back, we found out our night of concrete sleep was worthless and we were in the back upper deck of the arena. One particularly angry bear of a man started pounding on the plexiglass, threatening to tear the bearer of this bad news apart limb by limb. The rep behind the glass wasn't even the one that sent us home when the computers jammed...we had to calm the guy down (with beer...that always sometimes works).
For Zoo TV, we heard about the show on the radio in a dorm room at 2 am the night before/day of the tickets going on sale. 4 of us staggered down to Dillards at the Tucson Mall and stood around until 6 am, where they gave us lottery numbers and told us to come back at 10 and get in the order of the numbers. We used the best of the 4 of us, which was like 9. That got us section 7 on the floor, which was middle, 2nd set of 20 or so seats, 4th row. It was pretty much middle right facing stage and about 24 rows back on the Sun Devil Stadium grass. Not bad.