There's been a bunch of press recently about what they used to call the San Diego Comic-Con, but now call Comic-Con International:San Diego, talking to other cities, such as Anaheim and Las Vegas, about moving there, because the SD convention center has maxed out its room.
Mark Evanier, who has been to every Comic-Con over the past four decades, doesn't think the con is going anywhere. (http://www.newsfromme.com/archives/2010_02_26.html#018576) And I agree. Comic-Con has been very good to San Diego; taxes from its attendees have helped redevelop the city's downtown, which is light years better than the bordertown cesspool it was when I and my family went to my first Comic-Con 30 years ago. San Diego isn't going to let the con go without a fight.
The more likely result is that the con will spread out in town itself. When I went there in 1980, the con events were distributed between the (much smaller) old convention center and the surrounding hotels. The con is heading back in that direction, especially with the new hotel that sits next to Hall H. That will probably require more shuttle buses, and won't do much for downtown traffic. But people like San Diego, which is gorgeous in the summer; and they like the Convention Center, which sits right on the bay and has spectacular views for those who pull their heads out of the dealer's room once in a while. I think the con's going to stay there for some time.