[This API is no longer available. This page is maintained purely for historical and research purposes. Those interested in adding Kayak's search functions to their website may be interested in their search widget, documented here.]
From their site: The search API is an HTTP interface that lets you, the programmer, integrate kayak.com searches and results into your web site, desktop application, or mobile phone application. If you can make an HTTP connection, you should be able to use this API.