Right now I think noone is building such a thing because it's not clear that the feds won't shut it down as soon as they notice. This isn't a big problem for users of virtual currencies, but people running an exchange market could be prosecuted on money laundering charges at the very least.

If there were legal precedent that virtual currencies are fully legal to exchange with USD, I think you'd find that exchanges would mushroom overnight, and that virtual currency (particularly Bitcoin) would have a huge spike in value.