As idiotic as it might sound, I think if the IRS discovered me trading an in-game currency, they'd want a piece of it.  Whether they'd throw me in the clinker for not reporting it...probably not, but I think legally you have to report any gains or losses, sales, all of that.  But I'm not a tax lawyer.  Bitcoin is an asset, and there's capital gain/loss considerations that have to be kept track of if you don't want to run afoul of the tax collectors.

These exchanges, except for the US ones where I know they're regulated, are all shady to me.  It never surprises me when one turns scam, like bitfinex.