ESX 3.5 U1 10Gbps Ethernet Document from VMware

This is a link to a VMware published technical document outlining tests of ESX 3.5 U1 (applies to recent U3 as well) w 10Gbps network adapters. Below is an excerpt from the Conclusion section:

Conclusion
The results presented in the previous sections show that virtual machines running on ESX 3.5 Update 1 can efficiently share and saturate 10Gbps Ethernet links. A single uniprocessor virtual machine can push as much as 8Gbps of traffic with frames that use the standard MTU size and can saturate a 10Gbps link when using jumbo frames. Jumbo frames can also boost receive throughput by up to 40 percent, allowing a single virtual machine to receive traffic at rates up to 5.7Gbps. Our detailed scaling tests show that ESX scales very well with increasing load on the system and fairly allocates bandwidth to all the booted virtual machines. Two virtual machines can easily saturate a 10Gbps link (the practical limit is 9.3Gbps for packets that use the standard MTU size because of protocol overheads), and the throughput remains constant as we add more virtual machines. Scaling on the receive path is similar, with throughput increasing linearly until we achieve line rate and then gracefully decreasing as system load and resource contention increase. Thus, ESX 3.5 Update 1 supports the latest generation of 10Gbps NICs with minimal overheads and allows high virtual machine consolidation ratios while being fair to all virtual machines sharing the NICs and maintaining 10Gbps line rates.

http://www.vmware.com/pdf/10GigE_performance.pdf