Job postings on Microsoft's careers site indicate that the company is working on bringing Skype to the Xbox platform. The postings are for various engineering and management positions in London, and the successful applicants would be working as part of the apparently newly-formed Skype Xbox Engineering Team. There's not much to glean about the form the app would take, although the ad does say that experience in Metro style apps for Windows 8 would be beneficial. The Xbox 360 itself is never mentioned — rather, Microsoft says that it is "building the next generations of our products and technology," so there's no indication of when we might start to see the fruits of this development.
Since buying Skype for $8.5 billion last year, Microsoft has expressed plans to integrate Skype into Windows Phone and its other services, and just this week we saw job listings for development of a Skype web app. Of course, the Xbox 360 has its own options for video chat via Kinect or the Live Vision camera, but Skype integration would certainly be welcome — the PSP comes with a Skype client built into its OS as standard, so it wouldn't exactly be unprecedented, either.