VMware has released the public draft vSphere 5.1 hardening guide

When implementing virtualization technology, organizations must ensure that they can continue to maintain a secure environment and meet their compliance obligations. To do so, you will have to evaluate risks that might affect protected information and mitigate those risks through risk-appropriate standards, processes, and best practices.

The draft version of the vSphere 5.1 Hardening Guide provides guidance on how to securely deploy VMware vSphere 5.1 in a production environment. The focus is on initial configuration of the virtualization infrastructure layer, which covers the following:

• The virtualization hosts 
• Configuration of the virtual machine container (NOT hardening of the guest operating system (OS) or any applications running within)
• Configuration of the virtual networking infrastructure, including the management and storage networks as well as the virtual switch (but NOT security of the virtual machine’s network)
• VMware vCenter Server, its database and client components
  
  Public Draft (Revision A) of the vSphere 5.1 Security Hardening Guide
  http://communities.vmware.com/servlet/JiveServlet/downloadBody/21732-102-2-28939/HardeningGuide-vSphere5-1.Draft%20RevA.xlsx