Robin Davies commented on my earlier post about custom controls in WPF, and he makes a pretty good point, I think, that perhaps WPF will help drive the presentation layer to have more designer involvement in the layout and presentation of rich clients, similar to how they now "own" the presentation layer (HTML) on Web Applications. That seems like something plausible, with the WPF theme, style and templating mechanisms filling the roles now handled by CSS in web application (it's not a perfect analogy, but it should suffice for the argument here).
When, and how this might take off, I don't know, honestly. I haven't taken a look at Expression yet, so I don't know how that will pan out, but from my original experience with HTML, it will probably take a while. Here at home it took a couple of years, at least, after doing Web Application work became mainstream, for graphics artists and designers to start doing HTML. Before that, they would just mock up the screens in Adobe Illustrator or a similar program and then just cut up the graphics for you, but it was our job as developers to create HTML that would render like their illustrator mock-up. So, it might probably be a while as well for graphics designers and artists to start coding XAML styles and templates, I guess :-)
That said, I do not think that fully answers my questions regarding reusability, but it's a start.