I going to have to take a look at this Composite UI Application Block from Microsoft's patterns and practices group.

It actually looks very similar (at least from a 10k foot view) to the architecture/design implemented by Omar Al Zabir in a writeup over at CodeProject.

I admit, I've never been much of a WinForms guy, as development of Windows clients has never really interested me (partially because of what a pain in the ass it is to work with WinForms compared to WebForms (XHTML markup just makes more sense for UI layout and presentation, at least in my opinion)), but I think it's probably worth my time to take a good look.