I wanted to be able to use my already existing IIS server, instead of VS2010′s built in development server, but I kept getting “503 Service Unavailable” when I had the application pool set to .NET 4.0.

Looks like mark @ fooberry.com had the same issue:

“After recently installing Visual Studio 2010 Beta 2 and creating a .Net 4.0 site, I tried to step out of Cassini and into IIS only to get “503 Service Unavailable” and a killed worker process. Checking the event viewer I found the following errors…”

… read more here.

Rebooting now… fingers crossed!