Here are some notes from the research I've done on this subject.
1. The last World's Fair (now called Expo) in the US was 1984 in New Orleans
2. The 2012 World's Fair is in Yeosu, South Korea http://www.worldexpo2012.com/
3. There seems to be no rule to the amount of years in between each Expo. Some are four years apart, some are only one year apart. Sometimes you even have two in the same year!
4. The next Expo is in Milan, Italy 2017 (I suggest a book club field trip. Let's start those fundraising car washes now!)
5. Unlike 160 other nations, the United States is no longer a member of the Bureau of International Expositions. Congress withdrew funding in 2001.
6. This is the logo of the B.I.E. Don't ask.