An article in Forbes postulates which countries billionaires could purchase, factoring in their estimated worth and the countries' GDPs. On the list: Bill Gates, Warren Buffet, George Lucas, Zambia, Haiti, and Belize.
Update: A valid point to make here is that a billionaire's income isn't an accurate measure of their ability to "purchase" a country based on their GDP, especially if you think of the GDP as the equivalent of rental income. For instance, if a person's net worth is $9 billion, which is equivalent to the Bahamas' GDP, that doesn't mean the billionaire could buy the islands. He or she could only rent it for a year, theoretically. Then again, the idea of countries being up for sale, and individuals purchasing (or renting) them, is a somewhat silly premise. (thx, ian)
Update: Perhaps purchasing countries isn't such a silly premise after all. In 2003, the entire principality of Liechtenstein was up for rent. The tiny country, which borders Switzerland and Austria, attempted a "rent-a-state" program sponsored by Xnet. The idea was to draw attention to the tourist-friendly charms of Liechtenstein by essentially "renting" the country's hotels, restaurants, and sports stadiums en masse. (thx, colin)