I am not a tax expert, but I imagine that existing IRS rules still apply. For instance, if you were a contestant on a gameshow and won a new car worth $50,000, the IRS treats that as $50,000 in income. Likewise, if someone was paid $50,000 worth of Bitcoins, I imagine the tax obligation issue would be the same. Even if there is some sort of loophole at present, I would expect that an absence of tax revenue for the government would cause them to wake up and force employers to disclose to the government Bitcoin compensation...and then tax it...either in local currency terms or in Bitcoins...