I just watched Laurent Bugnion’s excellent talk about MVVM (Model-View-ViewModel pattern) from the MIX10 conference. In that talk he shows a cool trick when he attach the Visual Studio debugger to Expression Blend which lets you debug your design time code. It’s often really hard to figure out why your design time data doesn’t show up when using the MVVM pattern and this trick really helps you.
I had some problems to get it to work thou. First i thought it was some stupid limitation in Expression Blend for Windows Phone but after a while i realized that i had to choose “Manged (V4.0) code”. The default in my version of Visual Studio where “Manged (V2.0, v1.1, v1.0) code”. When i changed this everything worked perfectly. A more step-by-step description of how this works can be found here.