It would be dealt with just like any sale for something besides cash. It happens all the time, however tax authorities would have a hard time evaluating it. Its possible that they may treat the transaction as a gift rather than sale because arguably bitcoins aren't anything. In legal parlance, bitcoins may not be "consideration"
http://en.wikipedia.org/wiki/Consideration. In which case the seller would have large tax debt as gifts are taxed at a high rate in the US.
Another possibility is that it maybe treated as an exchange for securities such as stocks or bonds, however bitcoins don't quite qualify as securities in the traditional sense so it will just have to be hashed out (excuse the pun) once somebody tries it. There was a legal review done by a yale law student about bitcoin not too long ago, he would be the best to ask.
