Redirecting site with form authentication

Hi
I have an asp.net site with form authentication and default.aspx as the starting page. I have added an additional default.asp file to the site root folder which redirects to an external mobile site if needed otherwise continues to the main site. Problem is even though default.asp is higher than default.aspx in the ‘default document’ order for the site in IIS7.0, the default.asp never gets called when site is accessed by domain alone such as www.mydomain.com. However when default.asp is called explicitly like mydomain.com/default.asp it works fine.
How can I ensure that the default.asp page is the first page called when the user accesses the site via domain name such as www.mydomain.com?
Thanks
Regards