VMware has released a draft of the vSphere Hardening Guide, and are encouraging feedback from the community.  It is broken into multiple pieces.
Part 1: Introduction
Part 2: Virtual Machines
Part 3: Host
Part 4: vNetwork
Part 5: vCenter
Part 6: COS

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8 new patches have been released for ESX 3.5 only.  These do not apply to ESX/i 4 or older versions of ESX/i 3.x.
ESX350-200908401-BG – General
ESX350-200908402-BG – Critical
ESX350-200908403-BG – General
ESX350-200908404-BG – General
ESX350-200908405-BG – Critical
ESX350-200908406-BG – Critical
ESX350-200908407-BG – General
ESX350-200908408-BG – Critical