Yesterday Microsoft announced kb2288515 about Agent states in Operations Manager and System Center Essentials
I believe this is a very important kb which must be in the kb library of every SCOM admin.
An agent, management server or gateway can have one of the following states:
Healthy – green check mark
The agent/management server is running normally.
Critical – red check mark
There is a problem on the agent/management server.
Unknown – gray agent. The check mark and agent name are grayed out.
The health service watcher on the root management server (RMS) that is watching the health service on the monitored computer is not receiving heartbeats from the agent anymore. It had been receiving them previously (and it was reported as healthy), but now it is not. This also means that the management servers are no longer receiving any information from the agent at all.
The computer running the agent might be down, or there might be connectivity issues. You can find more information on the Health Service Watcher view.
Unknown – green circle with no check mark
The status of the discovered item is unknown. There is no monitor available for this specific discovered item.
Resolutions can be found here: http://support.microsoft.com/kb/2288515