IE9 RC fixed the “Internet Explorer cannot display the webpage” error when running an ASP.NET application in Visual Studio – which is good, as it’s the reason I flagged the beta and decided to wait for the proper release (although really I should have known it was related to IPv6 as I’ve seen something identical a long time back with Firefox and its IPv6 support).
It’d be interesting to know if this was also an issue with the recently released IIS Express, or if it was just Cassini.
You also might be interested in reading about Fiddler and the IE9 Release Candidate.