I've seen a great video (Mix09) of how easy it is to quickly sketch a web site using Blend 3 and specifically Sketch Flow.
What I don't dig is how to mobilize this for ASP.Net. Blend lets you "create websites and applications based on Silverlight 3" but how do you use Blend's output with ASP.Net?
The best way is to create the site in VS 2008 as a Silverlight, Silverlight Navigation or Silverlight Business application site. Then you can load the same site in Blend to do all the cool blend stuff to the site - build the site in Vs2008 and publish it to the web.
Blend/SketchFlow doesn't output to an ASP.net site, it outputs XAP(silverlight) and hosts it in an HTML file and includes Silvelright.js. If you want it in the ASP.net just copy it into an ASP.net website.
However, Sketchflow doesn't support RAD prototyping of HTML/ASPX pages.
When they said creates websites based on Silverlight3, they obviously meant, you had to include Silverlight, I think that is very obvious.
If you want some tool to prototype ASPX pages, use Expression Web or Visual Studio Design View or something else, not Blend.