MVC routing not ignoring routes

Question

I've seen numerous blog posts that indicate that, to use ASP.NET Webforms within an MVC application,simply add a line like

            routes.IgnoreRoute("{resource}.aspx/{*pathInfo}");

to the RegisterRoutes routine, and the aspx files will be ignored and served up in the conventional manner by IIS.

However, it's not working. We are running on an IIS 5.1 installation on XP (SP 3), and have the wildcard mapping to aspnet_isapi.dll implemented in the IIS configuration so that all requests go through isapi. But an aspx file ("Diag.aspx") in the root of the application gets a 404 when it's accessed. If I move that file into Content directory (which has had the wildcard mapping turned off), then it gets served up as expected when accessed as /Content/Diag.aspx.

What am I doing wrong here?