I am working in a startup that aggregates rental car rates, similar to Tripeedo using OpenTravel. Its all XML based, and can be a PITA--but the result are a better product and a better user experience.
OTA is gaining real traction. By my count about 1/3rd of the companies listed on Tripeedo are members of OTA, while many more unofficially support some derivative of it. Most of these companies don't have open API's--they are very private and require NDA's and partnership agreements.
There are systems in place to handle use-cases like this. API's are in place for travel agents to search available rates and creating bookings. This is essentially the same concept.
Kayak is justifiably being blocked because they're scraping sites. If they had partnership agreements in place this wouldn't happen.