Security :: Forms Authentication And IIS6 Default Application Mapping?

We're developping an application that uses forms authentication and URL rewriting (www.urlrewriter.net). In order to make extension-less URL rewriting possible we had to add a default application mapping for the asp.net aspnet_isapi.dll.

The problem we're experiencing now is that when the website is accessed by http://www.myapp.com/ then the aspnet_isapi.dll gets the request and forms authentication forwards it to our login site, so the user gets redirected to http://www.myapp.com/login.aspx?ReturnUrl=%2f instead of having default.aspx displayed (which is allowed to anyone).

Here's the part of web.config dealing with forms auth:

<system.web>
<authorization>
<deny users="?"/>
<allow users="*" />
</authorization>
<authentication mode="Forms" >
<forms slidingExpiration="true" cookieless="UseCookies" defaultUrl="default.aspx" loginUrl="default.aspx" name="gzfb_site_test" timeout="525600"></forms>
</authentication>
</system.web>
<location path="default.aspx">
<system.web>
<authorization>
<allow users="*"/>
</authorization>
</system.web>
</location>

I did quite some research on this problem but didn't find any solution. Is this scenario possible at all, or does default application mapping and forms auth using <deny users="?"/> interfere by design?

P.S. the problems only manifest when running on IIS 6, the ASP.net Development Server handles it without any problems