I went to a presentation by Expedia with Chris Hanrath today. One of the topics covered was what Expedia is doing that is experimental. Expedia, like Google, now has a Labs division to play around with interesting ways of evolving their core business: http://labs.expedia.com/
Seeing their Flights app I said to Chris - who interned at Yahoo this summer - "that is clearly YUI." Nerds that we are, we both viewed source simultaneously. Sure enough, Expedia is using YUI for their labs. Their static resources all have a timestamp parameter attached which is usually a good hint (though not proof) that the document was created in Rails. A little more searching turns up the HTML comment <!-- here is the Rails 'default' block of JS -->
Very cool!