Your view that the purchase of bitcoins for fiat currency constitutes a barter exchange because you could possibly trade them for a product or service elsewhere after receiving them is not a valid argument. First barter is defined as trading goods or services for goods or services. If an exchange only deals in bitcoins for fiat currency no barter exists. Any bartering done later by that individual with the bitcoins they purchased is outside the realm of the exchange. If the person makes a profit it is their responsibility to report income to the proper taxing authority.
Having read a different topic, a person posted that he had spoken to a securities lawyer, and they had reached the conclusion that public trading on the "Global bitcoin stock exchange" (and hence any exchange in the US dealing with securities) would subject it to SEC and relevant laws. A security is defined as:
"(3) the term security means
(A) a note, stock certificate, treasury stock certificate, bond, treasury bond, debenture, certificate of deposit, interest coupon, bill, check, draft, warrant, debit instrument as defined in section 916(c) of the Electronic Fund Transfer Act, money order, travelers check, letter of credit, warehouse receipt, negotiable bill of lading, evidence of indebtedness, certificate of interest in or participation in any profit-sharing agreement, collateral-trust certificate, pre-reorganization certificate of subscription, transferable share, investment contract, voting trust certificate, or certificate of interest in tangible or intangible property;"
US Code Title 18 § 513. Securities of the States and private entities.
Considering the fact that a securities lawyer has a far superior legal comprehension than myself, I concur that Bitcoins are not considered Barter. Also, it cannot be considered both a barter and a security, which I fallaciously argued earlier. However, as Bitcoin is a "debit instrument as defined in section 916(c) of the Electronic Fund Transfer Act" ("As used in this section, the term "debit instrument" means a card, code, or other device, other than a check, draft, or similar paper instrument, by the use of which a person may initiate an electronic fund transfer.") any Bitcoin exchange based in the U.S. is subject to the SEC and relevant laws. And, of course, in the globalized financial business sphere, all stock exchanges in the world are subject to similar legal provisions and all defintions of "securities" are likewise or very similar, and due to "harmonization" the laws and definitions are being aligned continuously.
So all Bitcoin exchanges, wherever situated are subject to the exchange laws of the host nation, as Bitcoin is a kind of "security", I contend. Exactly which tax laws that are applicable is beyond my current knowledge; although the only point that we have agreed upon so far is that gains on Bitcoin trades are subject to some kind of tax.