Todays details: the Web Application Site builder (aka Designer ) is a .NET winforms based application (not COM). It is a site builder tool for creating ArcExplorer Style web applications for your own sites. The output can then be reloaded in the builder for some customizations or opened in Visual Studio.NET and work directly with the web controls and underlying OM. The ArcExplorer Web that was shown is an ASP.NET web application that sported some of the new web controls and user experience enhancements that the .NET Server team (one of my teams) have been working on and that I eluded too in my AJAX postings. Yes, the .NET Server team is setting the pace for the richer user experience and web client framework development work at ESRI.
Sweet! We do almost all our programming in .NET and VS.NET so anything to make that easier is welcome.