A bit more thought should have gone in this question (and the answers) before posting them.

The documentation says that this feature "Requires the 64-bit edition of Windows Server 2008 Datacenter or the Windows Server 2008 Enterprise Edition for Itanium-Based Systems operating system". So while Windows Server 2008 Enterprise Edition x64 is definitely not on the list (sorry, Dr. Diana Dee), Windows Server 2008 Enterprise Edition IA64 is.

The answer "Windows Server 2008 Datacenter Edition x64", that I chose (and currently 30% of all participants) is the least incorrect of all options - but it's NOT correct!