Am I correct in reading that the Edge Transport Server role must also be installed on a Server OS? I'd sure like to run it on an XP machine.

For any role, you need one of the following OS: 64-bit edition of Windows Server 2008 Standard with Service Pack 2 (SP2) 64-bit edition of Windows Server 2008 Enterprise with SP2 64-bit edition of Windows Server 2008 R2 Standard 64-bit edition of Windows Server 2008 R2 Enterprise Exchange 2010 System Requirements http://technet.microsoft.com/en-us/library/aa996719.aspx