I've seen it been done with tracksuit bottoms... ones with an elasticated waistband... and not CK boxers, cheap ones from Tesco or M&S or something, plain grey... it's not a pretty site...
Oh, and I did see one guy almost lose his jeans in the street, he had to catch them as they fell to his knees

my mother was walking with me behind him saying 'they're going to fall.... they're going.... going.... gone

' for a while, she was trying to get me to bet on how long they'd stay up for

. And as soon as he pulled them back up, they fell right back down to show off his pants -_-
Yeah, it's a silly fashion trend which will hopefully pass... but to be honest, I prefer the pants-above-jeans look to those hats which my mum refers to as 'full nappy hats'.