Alex Bakman offers 10 tips on his blog site for helping improve VMware ESX security.  He writes:

At VMWORLD 2006, I offered these 10 recommendations to improve ESX security in your shop.

1. Use Firewall and Antivirus software for COS. Just as in any other operating system, this provides basic protection
2. Use VLANs to segment the physical network so only machines that are required to see each other are able to do so
3. When installing ESX, use security=high
4. Do not allow root level access over SSH and use secure commands
5. Disable all unnecessary services in console OS
6. Use VirtualCenter to help you manage granular security access
7. Stay current with ESX patches
8. Harden Guest Operating Systems
9. Control User Level Access using VirtualCenter
10. Document and monitor configuration changes in your environment, especially changes in security settings

You can download a PDF of my presentation materials here.

Read his blog, here.