Based on popular demand, here is a quick sample (WPF only though it would be very easy to port this as-is to Silverlight - currently busy with Blend 3 work and all the cool newer features we are adding to Blend 3) that will allow you to get started on the following new extension points we have added to Blend 3:
a) The new way to specifying metadata for your controls
b) DefaultInitializer that allows you to set properties when a control in instantiated from the Blend asset library
c) Custom context menus
d) An adorner for the control
Let me know if you wanted some samples for any specific scenarios you might be interested in.