Control authentication dialog in SharePoint 2010 using SSO

Question

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

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

Any help would be greatly appreciated.