I though of sharing this tip since I've found this extremely useful and have used several times. You might have been in situation where you want to start troubleshooting the communication between VC & the hosts.

When you see an error message in VC something like "Unable to access the specified host. It either does not exist, the server software is not responding, or there is a network problem." or for other reasons want to ensure the communication between the VC & ESX host is Ok, execute the 2 commands below:

# service vmware-vpxa status
# service mgmt-vmware status

If you see anyone of the above services stopped, start the service using:
# service vmware-vpxa start
# service mgmt-vmware start

Similar troubleshooting steps are listed at the vmtn: http://www.vmware.com/community/thread.jspa;jsessionid=AA458F1B4D71FDF5DB9E9C45CFF52729?messageID=576253&#576253