I have been trying to do this for a very long time.  Unfortunately the
docs are still only covering VS 2003 on MSDN, so now that I just
figured it out I put together a tutorial to share.  You can find it on
my blog.

http://brennan.offwhite.net/blog/2006/08/30/design-time-debugging-aspnet-20-in-visual-studio-2005/

I hope this helps anyone looking to debug design-time support.

I also have links to helpful authors who write about the
CompositeControl and CompositeDataboundControl.  I really like working
with them, but there are just a few areas where the documentation does
not explain.

Brennan Stehling

Really Stuck on a Canlendar Control PLEASE HELP!!!
Need some advice on a GridView control.
Events for programatically added controls
UserControl values lost on PostBack
Menu control. Clickable area
Custom Server Controls and javascript
CustomWeb Controls and Events
I want to know what is a Functional Specification Document?
Clikable rows in Grid View
Custom Collection Property Persistence