There's a bucket that gets passed around for the whole General Admission line. ;-)
At least here in the States, GA lines were fairly accomodating when it came to leaving the line to use the bathroom, or to run and get food, or whatever else (provided that it wasn't for too much of an extended time).
A wise thing to do is to make friends with those around you in line - that way, should you have to leave for a bit, you can give everyone a heads-up so that there's not any issues when you return. Just don't be that guy who says he'll be back and then disappear for three hours!
When you're actually on the GA floor, it becomes more of a risk to leave your spot, I suppose. I've never left my spot once I got to the floor for fear of not being able to get it back, and that's the advice I would give to anyone, most especially if you end up close to the stage.
If you're looking to get a broader slice of people's line experience, I'd recommend checking out a site like u2tours.com - you can ask on the forum over there or join their mailing list...
Hope this helps.
