In upgrading from SharePoint 2007 to SharePoint 2010, one of my InfoPath forms would not return the XML data for a form field. The error I was getting was:

XML data that is required for this form to function correctly cannot be found

If I previewed the form in InfoPath 2010, I had no issues. It was only on the site that this error would occur. 

Here is my original data connect XML link:

http://sharepoint/_vti_bin/owssvr.dll?Cmd=Display&List={4B14953C-5E0D-45A9-B99A-6967F8FB8A73}&XMLDATA=TRUE

This worked fine in SharePoint 2007, no issues. In SharePoint 2010 apparently, it needs to look like this:

http://sharepoint/_vti_bin/owssvr.dll?Cmd=Display&List={4B14953C-5E0D-45A9-B99A-6967F8FB8A73}&XMLDATA=TRUE&noredirect=true

After that, it works fine. 

reference: http://msdn.microsoft.com/en-us/library/ms478653.aspx