Ask YC: How does Kayak or Mobissimo work?

I'm looking to develop a simple fare, hotel and car rental tool similar to kayak.com or mobissimo.com.

Both sites search hundreds of other travel sites, hotel chains and airlines, show the end-user the aggregated results in a nice way and then direct them to the original sources passing an affiliate id code so they can profit.

I assume that the majority of the sites they pull information from don't provide any kind of APIs.

Does anyone know how they might be able to pull this off?

I was wondering if there is a web service out there that gives you access to query the majority of these sites from an central API or if they are just having to scrape hundreds of sites individually.