There is a new bread of travel sites cropping up that pose a serious threat to the current leaders in the online travel space. Sites like Sidestep and Kayak are significantly different than the "old school" travel sites like Expedia and Travelocity in that you do your price search through their site, but book directly with the airline or hotel.
With more and more airlines and hotels guaranteeing the lowest rates when booking directly, consumers will gradually become savvy enough to realize that there is little reason to book through an aggregator like Expedia. According to a recent report by HitWise, the market share of visits to travel meta-search engines Kayak and Yahoo! FareChase increased by over 70% from May 2005 to May 2006.
Further diminishing the value of the aggregators, many airlines such as Continental and hotels including Hilton, Starwood, and Marriott go beyond just the best rate guarantees and offer perks such as upgrades or bonus loyalty points for booking directly. If you read through TripAdvisor, you'll also find countless tales of people who booked through sites like Expedia only to find they had problems once they arrived because they did not book direct.
There is also a pretty revolutionary new site still in beta called FareCast.com that tracks average airfare prices between major cities and advises you on the best time to purchase based on recent trends. Sites like this give the consumer so much more buying power than they have ever had before.
The one area that companies like Orbitz and Expedia can fight back is in packages. A recent search for airfare + hotel for an October trip to San Francisco if booked directly through Marriott and United Airlines would have cost me about $1250, while a package booked through Expedia would have only cost $775, a savings of 38%. I tested several other travel scenarios and found nearly all of the packages to be considerably cheaper than buying direct.
The late 90's saw the virtual extinction of offline travel agents when people discovered online travel booking, and I think it will only take 3-5 more years before sites like Orbitz, Expedia, and Travelocity also border on extinction.