It is actually not illegal to transact commerce in the U.S., using foreign denominations. It is illegal to print dollars, or to print a competing currency here -- that's probably one of the points they'll use to go after BC.
Mack
This stuff gets confusing! If you follow the links I've provided you'll find that it might not be clear WHAT Bitcoin is, and this might be why we don't see any definitive opinion on this from the US Treasury, inasmuch as they might not want to make a mistake that result in a court ruling in favor of Bitcoin.