Intel processor specifications list the virtualization feature vt-d as a processor feature of the xeon 5500 series but not of the 5600 series. Vt-d is needed for io virtualization.
How does vt-d relate to intel processors, there is no information.
I just learned the answer from an intel marketing manager and thought I should document it here for the common benefit.
vt-d is an Intel chipset feature which enables iommu / directed io in virtualized environments, eg. xen. It is not a processor feature like vt-x. vt-d is neede for io passthrough to guests, like PCI passthrough.
A complete answer obtained from an intel marketing manager is given below.