This post is mostly a self-serving bookmark, but if you somehow landed here and are having issues with ASP.NET MVC 2 URL Routing (or routing in web forms if you’re using it there!) and are running on IIS 7 or 7.5 then have a read through the KB article below. I found it after much head scratching and it fixed my issue :-)
http://support.microsoft.com/kb/2023146 : MVC 2 and ASP.NET 4 Web Forms applications that use URL routing might return HTTP 404 errors when they attempt to process extensionless URLs on IIS 7 and IIS 7.5.
Originally posted by Stuart Leeks on 24th August 2010 here: http://blogs.msdn.com/b/stuartleeks/archive/2010/08/24/extensionless-urls-not-working-with-asp-net-mvc-2-on-iis-7-7.aspx