Some of you probably find out that the beta version of IE9 seems to cause some issues when we want to run an ASP.NET or Silverlight in debug in Visual Studio. In reality, there is no issue. In fact, it’s IE9 who is too fast, compare to the Web Development Server (also called Cassini) which take longer to instantiate. A friend of the .NET Montreal Community Étienne Tremblay found a workaround for this issue. He wrote a post about his solution: http://geekswithblogs.net/etiennetremblay/archive/2010/10/07/ie-9-cassini-and-the-dreaded-dns-error-or-page.aspx
I tried it and it work fine for me!