A job listing for a Google Senior Account Executive, Travel Vertical made a ZDNet Blog think that Google intends to launch a booking feature, Google Travel. They even imagine how Google Travel would integrate with Google Videos (to promote destinations), Google Talk (click-to-call feature for listed hotels, restaurants) and Google AdSense."Your main responsibility is to drive new business revenue growth with our Fortune 1000 advertisers in a specified vertical [my note: travel vertical] in one or more regions" the job listing mentions.
For now, when you enter two cities in your query, Google links to third-parties travel planning and flight-booking sites (Expedia, Hotwire, Orbitz), but the experience isn't very good: these flight-booking sites have slow search engines and intrusive ads.