Anonymous access to web-based forms using UDC

I've got two zones:

Intranet (NTLM)
Extranet (FBA with anonymous access enabled)

There is an infopath form that connects to a web service. The browser form works when conneting through NTLM and FBA users that are authenticated. However, anonymous user receive an error. Any ideas on how to allow anonymous users to use the form?

The following is the authentication line from the UDCX file

<!--udc:Authentication><udc:SSO AppId='' CredentialType='' /></udc:Authentication-->

[543 byte] By [PiroozJ] at [2008-1-25]