Hi have a pluign for this and documentation - please feel free to use:)
Here is some documentation:
http://www.op5.com/how-to/monitoring-vmware-esx-3-x-esxi-vsphere-4-and-vcenter-server/
And here is the plugin:
http://www.op5.org/community/plugin-inventory/op5-projects/check-esx-plugin
/ J