For archive, I just wanted to inform that Beta 9 has added support for modal confirmation dialogs (via MessageBox and MessageBoxButton).
By the way, even though it is not modal, users may be interested to know that Beta 10 has added support for ChildWindows. To use them, right-click your project in the Solution Explorer, and click Add -> New Item -> C#/XAML for HTML5 -> Child Window. You will find the documentation at: http://cshtml5.com/links/how-to-use-the-child-window.aspx
You can download the latest version of the product from:http://forums.cshtml5.com/viewforum.php?f=6