It looks like Solera Networks built a virtual tap, as I hoped someone would. I mentioned it to Solera when I visited them last year, so I'm glad to see someone built it. I told them it would be helpful for someone to create a way for virtual switches to export traffic from the VM environment to a physical environment, so that a NSM sensor could watch traffic as it would when connected to a physical tap.
This picture describes what it does:
You can read more in this news post and product description. You can download it here. The V2P Tap requires ESX Server, which I do not run. If someone with ESX Server downloads the V2P Tap, please let me know how it works for you.