Right, Intel's an ass about product segmentation. But most hypervisors don't offer the option of doing any sort of PCI passthrough without a functional IOMMU, so they're not accidentally exposing anyone.

It would be sensible for hypervisors to require IOMMU for all PCI passthrough, but it looks like Xen allows it without IOMMU for paravirtualised guests:

"VT-d Pass-Through is a technique to give a domU exclusive access to a PCI function using the IOMMU provided by VT-d. It is primarily targeted at HVM (fully virtualised) guests because PV (paravirtualized) pass-through does not require VT-d (altough it may be utilized too)."[1]

[1] = http://wiki.xenproject.org/wiki/VTd_HowTo