I test my app only on local cassini web server, and I get strange thing.
One Index page works right in default manner, second shows 404, but when use full version Controller/Index – works normal. Only difference I found is that in first sample there is no slash in the end, and in second slash exists, but link that points on corrupted is without slash.
Can you show us your routes? Or even better: Try to use Phil Haack’s excellent (and essential) Route Debugger. It will tell you which route(s) are hit for a given request, or if no route is hit at all.
http://haacked.com/archive/2008/03/13/url-routing-debugger.aspx
Works only with MVC Beta though, not if you use any older Preview Release of MVC.