In this post, Max Paulousky has created a sketching GroupBox control in WPF.
Microsoft Expression Blend 3 has a great feature – SketchFlow. It allows creating gorgeous prototypes for Silverlight and WPF applications. But it has some disadvantages like shortage of controls. For example, there are no sketching GroupBox controls in Silverlight and WPF sets, sketching DataGrid (Silverlight and WPF) etc.