Here are a couple of links about the issues with detecting when a control is design mode (within Expression Blend or Visual Studio):
Second Stanza
Recent Posts- MissingMethodException and WaitOne
- Security Exception on Silverlight Client when Calling WCF Service
- “Resource not found for the segment …” Azure Exception
- Silverlight Color Picker Control
- Silverlight Modal Popup
- Resizing a Silverlight Control within the Browser
- Second Stanza Source Code Repository
- Detecting Design Mode in Silverlight Controls
