This is because if you want to rewrite to something without a known extension (.htm, .aspx, etc) you must tell IIS to pass all requests for every object to ASP.NET

You can do this by going to IIS, opening the properties dialog for your website, and on the virtual directory tab, click the configuration button.  Then you can add a new mapping - the extension should be "*" and the executable path should be "(.net frameworkdirectory)\aspnet_isapi.dll" - you can copy the path from the mapping for .aspx

If you are on a hosted server you will probably need to ask them to do this for you - some might not.

JK