OK, disclaimer, I actually have no idea if it will or not but in the tradition of bloggers everywhere I thought I'd do a bit of overstatement to grab some eyeballs. Plus if it turns out to be true I can claim psychic powers.
After looking at a lot of WPF and Silverlight demos it seems that something like the Zune software would be a perfect fit. It allows for a much greater degree of UI customization and "prettification" while still integrating naturally into the OS. Looking at apps like Vertigo's Family.Show or thirteen23's Harmony you see the richness of UI that can be achieved while maintaining a functional application. With the Zune trying to position itself as a social, interactive device you need a software interface that mimics that immersive, social, networked experience and currently the Zune software feels about as social as watching a midnight matinee alone in an empty theatre eating stale popcorn.
Another reason is that many gripe about the Zune software being written on top of Windows Media Player 11 and while the concept doesn't bother me the fact that the Zune team is tied to the WMP team for software improvements does.
The last big reason I see the Zune software being written in Silverlight is to solve the cross-platform issue. A lot of criticism is heaped on the Zune simply because it doesn't have Mac or Linux software and while it did take a year for iTunes to come to Windows after the iPod came out newer portable media players don't quite have that luxury. I haven't done enough research to know how deeply you can hook into the OS, as I imagine you'd have to do to enable some things like the Marketplace DRM and USB device syncing, yet if it is possible you'd have a very strong poster-child for the power of developing in Silverlight.
Of course I can also see the obstacles. WPF isn't proven yet, in fact I don't believe it's even hit 1.0 status and it requires .NET 3.0, which in turn makes for a larger install with more pre-requisites. When you're talking about a consumer level device you want the software to be as easy to install on as many computers as possible and using fresh, out of the oven technology isn't always the best way to ensure a 100% trouble-free experience. Plus the developers now have twice the work-load as they not only need to push the full power of the Zune but they also need to to wrestle with learning the ins and outs of a new tech.
Given all the obstacles though I think there would be a huge benefit in writing the next Zune software version in WPF or Silverlight, both at the Zune level for the degree of interactivity to be gained and for Microsoft as a way to showcase just what WPF can do.