Dan-
I wondered where you landed! Hope all is well.
Agents can go offline for any number of reasons, and, just like SEP, identifying a 'dead' agent vs a 'rogue' agent can be a challenge. My first suggestion would be to validate the offline agent is not a dead duplicate. Check page 257 on the Admin guide...it outlines options for automatically dealing with duplicate agents.
If it's not a duplicate agent, my next suggestion would be to validate the host is still on the wire...the quick and easy way is through a simple ping (if ICMP is allowed/passed in your environment).
If it's on the wire and it's not talking to the console...the agent is rogue and will require attention at the host level to get it back. Fortuneately, we haven't found it very common for an agent to go into a rogue state on it's own...it generally has some help from a server admin.
Feel free to ping me if you need anything.