I followed the following tutorial http://blogs.msdn.com/b/infopath/archive/2010/03/11/customize-an-external-list-form-in-infopath-2010.aspx
In the video he has a dropdown list that he populates using an XML document. I attempted to perform this operation using a Microsoft SQL Database connection and I'm able to setup the connection but, when I attempt to publish the form I receive an error that says, "Additional Data Connections not supported." After receiving this error I again tried using an existing list in SharePoint to populate the dropdown and I still receive the same error message.
I'm new to ECT custom forms and would appreciate any advice.