I just published onto CodePlex a new WPF project, an addin for Visual Studio 2010 which allows adding and exporting XAML code snippets and that is supported by both WPF and Silverlight editors. You simply need to right click inside the XAML code editor and select the command you need (insert or export). The addin uses the classic .Snippet file format and the code is saved (or read) as XML. I took this approach to maintain compatibility with the VS .Snippet format.
Screenshots, getting started instructions and the source code, in Visual Basic 2010, are available in the project workspace here.
Since the XAML editor does not natively support code snippets, differently from VB/C#, there are some limitations. For instance, I built a couple of dialogs so there are no popups like in managed languages and there are no replacements: the XAML code editor isn't able of recognizing them, so adding replacements in code would result in errors.
Of course there are several improvements for future releases, such as grouping commands into submenus, implementing XAML preview, popups instead of dialogs and other interesting things but this is just the first release and I would appreciate your feedback to understand if the project is useful before planning other features.
Alessandro