Although Mtgox may not be subject to US taxes, depending on where it is based, as many traders are from the US, and given that all countries are more than likely to have similar tax provisions (the U.K. does) the Inland Revenue Service defines barter as following:
Barter Exchanges
Bartering is the trading of one product or service for another. Usually there is no exchange of cash… Unlike one-on-one bartering, members of exchanges are not obligated to barter or purchase directly from a seller. Instead, when a barter exchange member sells a product or a service to another member, their barter account is credited for the fair market value of the sale. When a barter exchange member buys, the account is debited for the fair market value of the purchase.
Internet-based Barter
The Internet provides a new medium for the barter exchange industry. Pure Internet-based barter companies differ from traditional, organized trade exchanges in that they do not have a physical office. In modern Internet barter exchanges, there is an agreement or process in place to value goods and services exchanged, which is facilitated by the barter exchange for a fee. A barter exchange functions primarily as the organizer of a marketplace where members buy and sell products and services among themselves.
Trade Dollars
Barter exchanges have their own unit of exchange, usually known as barter or trade dollars. Trade dollars or barter dollars are valued in U.S. currency for the purposes of information returns. Trade dollars allow barter to take place between parties when one party may not have a simultaneous need or desire for the goods or services of the other members. Barter exchanges act as the bookkeeper for keeping track of trade dollars that participants accumulate. Earning trade or barter dollars through a barter exchange is considered taxable income, just as if your product or service was sold for cash.
Many virtual communities, including Second Life and The Superfluid comply with US tax laws, for above reasons. Any U.S. resident bartering is thus subject to above laws, or so I believe. If such laws exist in Japan, if that is where Mtgox have their offices, they must also abide by such laws, if such laws exist in Japan. Above reasons are bound to be one consideration that has led the Electronic Frontier Foundation to stop accepting Bitcoins. If Bitcoin is to grow, it must be accepted as tender by many vendors; this will obviously not occur until it is easy to comply with the law.
I am not at all against Bitcoin, rather I abhor the FIRE sector that set the economical agenda and hinder economic growth and subject people to financial hardship and causes starvation in some parts of the world. Also, the benefits of Bitcoin are intended to be an alternative to financial control and regulation. However, this precludes a widespread adoption. Furthermore, tax laws are only one legal impediment towards Bitcoins.
The Securities Exchange Act of 1934 (U.S. law again) defines an exchange as following:
The term "exchange" means any organization, association, or group of persons, whether incorporated or unincorporated, which constitutes, maintains, or provides a market place or facilities for bringing together purchasers and sellers of securities or for otherwise performing with respect to securities the functions commonly performed by a stock exchange as that term is generally understood, and includes the market place and the market facilities maintained by such exchange.
So Mtgox, and all other exchanges, are subject to all law (any country has similar provisions) that regulates exchanges.