I should note that the behavior of our 2010 and 2013 SharePoint servers is different.
InfoPath forms on the 2013 box do respond with the full domain / account name
Forms on the 2010 box only return the account name.
and for further consideration, it appears the 2010 box will take a userprofile request without a domain name on it, while the 2013 will not. it's almost like there's a "treat as default domain" type switch for the service...but I don't think I've ever seen that anywhere in Central Admin
means we're going to have to inspect a ton of forms when we move our db over.