It was at the end of St. Jimmy and beginning of BOBD, before he started singing. I seem to remember someone finding it by searching for "balls" in Green Day Chat so gonna try that now
Alright here we go, found the original post where someone searched for "balls", haha. This is BOBD from Manchester where he says the thing about splitting his pants:
Dunno if you actually see it happen though.
Well these came up on the search, they're from a different show where the mic wires got tangled so he pulled his pants down and ran off, really bad quality but you get the idea...
The start of this one
http://www.youtube.com/watch?feature=player_embedded&v=5wJIzzjvTDY