Hot off the presses...

Error 1789 when you use the LookupAccountName function on a computer that is running Windows 7 or Windows Server 2008 R2
http://support.microsoft.com/kb/976494/en-us

And you can request the hotfix online (no phone call)
http://support.microsoft.com/hotfix/KBHotfix.aspx?kbnum=976494&kbln=en-us