External facing website.  

Using Sharepoint 2010 SP2 + SSO using a trusted Id Provider (Salesforce). 
Users can browse anonymously. On login they are redirected to the IdP.
This works.

However the website hosts webservices that need forms authentication.

Once I enable forms authentication, on login users are prompted with a dialog asking them to choose between Salesforce and Forms Authentication.

Is there a way to suppress this dialog and use the the trusted IdP in all scenarios other than the webservice (which must continue to use forms?)

[This link suggests that you can make changes to the webconfig to suppress the dialog, but that had no effect][1]

   

    <security>
    <requestFiltering allowDoubleEscaping=”true”>
    <verbs allowUnlisted=”true”>
    <add verb=”OPTIONS” allowed=”false” />
    <add verb=”PROPFIND” allowed=”false” />
    </verbs>
    </requestFiltering>
    </security>


  [1]: http://blog.techgalaxy.net/archives/3118
Any  help would be greatly appreciated.