
I googled it a while ago (for fear of using it incorrectly and looking like a total numpty :D), apparently it's a shortened version of "woohoo! Loot!" as used by RPG gamers online when their character finds (strangely enough) some loot in the game. It seems to have spilled over into online life elsewhere as an expression of joy or triumph, pretty much 'woo!' with a 't' on the end. :)
I'm online far too much but I've only started noticing it a lot in the past 6 months, if that so I think it's just starting to catch on.