I'm sure loads have people have seen this already.
I have a silverlight usercontrol (mysilverlightpage.xaml), which contains a silverlight usercontrol (mylefthandnav.xaml). If both usercontrols use a global style (defined in my app.xaml), the designer blows up in the container page (mysilverlightpage.xaml), but is ok in the contained control (mylefthandnav.xaml).
The application however runs fine. If I however remove the style from my application resources (app.xaml), and have a local version of the style in each user control, then all is fine.