This article from Associated Content is talking about expected traffic at Airports this labor day. Chicago is expected to be the busiest Airport (as United's hub it gets huge traffic), and the USA's second most popular destination city.
It's a mistake to use Orbitz data to generalize about city popularity since the analysis is complicated by issues regarding Airport hubs, final destinations that are often different from Airport destinations, etc, but it's an interesting view into American Travel patterns.
