I'm using an old Xeon 5400 box that happens to support VT-d to run a virtualized file server and it works extremely well. This is on ESX 4.0U1, and it also worked fine pre-Update1. Never tried ESXi.

Motherboard: SuperMicro X7DWA-N
Chipset: Intel 5400
Card: 3ware 9650SE-16
Device IDs: 1004:13C1

Evidence that I'm not making it up attached.

Throughput to the array inside the VM is quite impressive. 460MB/sec reads in ATTO.