Custom Form field Control designer

I've created a few custom field controls to use with the form builder. When I click on edit to go into the field properties, it is just the basic listing of fields available in the advanced view. How do I go about implementing the control designer seen in all the out of the box form fields? Is there an additional interface to implement or a simple view to add?

You sir are potentially in for a treat!

http://visualstudiogallery.msdn.microsoft.com/381fde64-9898-4a5d-ae20-861e119bbf92/

Todays thunder update contains forms designer support