While specific details would require competent tax counsel, and there are a lot of specific details, the general rule is that if you receive anything of value that can be converted into dollars at a fair market price (which I use in a sort of technical sense, not just as a wishy-washy term, the IRS knows how to obtain a reasonable approximation of this value), the IRS will hit you in some form for that price. BitCoin has reasonably well-established values in dollars which can be used, should it become successful enough for the IRS to care it will be even more firmly established what the dollar value of a BitCoin is, and if nothing else the IRS can always assign a market valuation to the other side of the transaction, the thing being purchased/sold.

When you win A New Car! on the Price is Right, you better believe the IRS and all relevant local tax municipalities express no confusion on how to tax you for it. BitCoin would not be particularly different. Without a rigid wall completely separating the BitCoin economy from the greater global economy (in which the USD is only one player), the greater global economy will tend to efficiently price a BitCoin. ("Efficiently price" is a technical term, what it means in this case is that the conversion rate in some sense has a real existence and isn't just a polite fiction, though there's a couple of simple logical steps betwixt the raw definition and that conclusion.)

There is basically no escape from taxes, other than any that may be explicitly laid out in tax law.