I can see that the camp that develops Windows Phone OS is going to progressively overtake and meld with the software developers of Windows 7. Apple is following that model with iOS becoming integrated with Lion. Just a matter of time before tablet-like computers replace the traditional setup, especially at the office, where terminal style application access could be implemented.
However, XP is going to stick around, whether MS likes it or not. It's paid for, business owners are cheap, and most of their software for day-to-day runs just fine with it. When there are more SaaS options and apps for mobile devices that supersede the need for an XP machine running a bunch of local apps, then XP will be supplanted.