I have created a true MVC-pattern for ASP.NET that allows dialogs windows in ASP.NET just like you would do when using WinForms.
I am using a technique called server-side-blocked calls (server-side AJAX).
On my website there are four demos online and also a recording of a live-coding-example (executable desktoprecording).
The framework is freeware and is currently being applied in several projects.
It's real cool, please tell all your friends about it!
It has saved me a lot of developping time.