Access Web Service in Infopath Browser-Enabled Form

I am using one of the default web services that comes with MOSS to fill in the current user's name in an InfoPath form.  It works very well using the InfoPath client, however the same form viewed in a web browser returns the following error:

An error occurred accessing a data source.
An entry has been added to the Windows event log of the server.
Log ID: 5566

The web service URL is:
http://sitename/_vti_bin/userprofileservice.asmx?wsdl

I found some information online referring to using a UDC file to make this connection outside of InfoPath.  I used the convert button in InfoPath to convert the file to a UDC that is saved in a data connection library on the site.  However, this still didn't resolve the problem.

I also added in the following section before the end tag for "ConnectionInfo".

            <udc:Authentication>
                <udc:UseExplicit CredentialType="NTLM">
                    <udc:UserId>DOMAIN\USERNAME</udc:UserId>
                    <udc:Password>PASSWORD</udc:Password>
                </udc:UseExplicit>
            </udc:Authentication>

I still had no success with or without the domain prepended to the username.

Finally, I tried changing the access in the form to Full Trust.  I uploaded the form to the Central Admin, approved, and set the content type in my form library to match.  Still no luck there.

I have a feeling I must be missing something simple.  This works just beautifully in InfoPath, but the browser form keeps giving me the error.  

Has anyone had success in using web services in browser-enabled forms?  If so, what steps need to be taken to make this work?