In October 2013 VMware released vSphere 5.5 Hardening Guide. I think this is quite an important document to keep your virtual environment safe and secure.
It consists great recommendations for several areas of vSphere stack - VM, ESXi, vNetwork, vCenter Server, VUM, SSO, WebClient, vCSA. It is delivered in handy Excel spreadsheet with the description of vulnerability and (which is, in my opinion, the “killer feature”) PowerCLI reference how to apply security recommendation.
There are two files - changelog and hardening guide.