VMware has released their hardening guide for vSphere 4.0.

There is more than 100 guidelines to choose from, divided between

  • Introduction
  • Virtual Machines
  • Host (both ESXi and ESX)
  • vNetwork
  • vCenter
  • Console OS (for ESX only)

http://blogs.vmware.com/security/2010/04/vsphere-40-hardening-guide-released.html