Overall form layout is very very similar
Source code shows that this is about the only residual similarity (although why sites like Bing even bother including a DOCTYPE - especially a strict one - when they get their headers so borked, is beyond me).
My guess is that, at an early stage, some devs (probably at Farecast) took a quick look around the web, found Kayak and copied the overall layout of its submission form as a prototype. A screenshot of the prototype then got welded into some early requirements document, and this document then fell into the hands of the pointy-haired division. The pointy haired division then convenend an endless succession of meetings and extended lunches, during which they all agreed on absolutely nothing, except that the layout of the submission form Must Not Change (as pointy haired bosses are wont to do).
I'm sure anyone who has worked in software long enough has seen this happen: the product is eventually delivered, five years late and twenty billion dolllars over budget, running at one twentieth of its original speed, and with nine million lines of unecessary code behind it - but it it still sports a front end that took five minutes to come up with on day one.
The irony? Farecast's developers probably fired up Google and typed in "Travel bookings flights hotels", or some such.