I'm currently working on a MVM WPF application some parts of which may also run on Silverlight. We have some complex, user created, UI elements. The simplest way to persist these is as XAML (just using XamlReader/XamlWriter). In nearly all cases we can recreate the items as either a WPF or Silverlight instance simply by changing the XAML namespaces before the reader gets at it. Looking at the Win8 documentation it looks as if the same technique would work there.

I guess I understand the reasons but it would have been really nice if
*identical* XAML (i.e. assemblies) could have been used across all three environments.....

>XAML was originally developed by DevDiv. It's now baked into Windows 8 and is developed/supported by the WIndows team, Which IMO is where it belonged all along. It's also now easily accessible from C++ and Javascript.
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>FYI, what was once one WPF/Silverlight/XAML team is now three teams: Windows, WIndows Phone, and DevDiv.
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>>On a side note, I took a quick look at the Xaml based stuff for Win8 today and, AFAICS, most of it could almost be transported from a Silverlight version - makes you wonder why, when they already have a WPF and Silverlight implmentation, they needed to re-invent the wheel....