By the way, I did check & with asterisk, UNREACHABLE does indeed mean unregistered, and Reachable does mean registered. So I think I am on the right track.
I don't know that I agree with that statment. Asterisk determines whether or not an endpoint is "reachable" by the use of SIP OPTIONS message. If it recieves a "200 OK" in response from the endpoint, Asterisk knows that it is "REACHABLE."
A SIP endpoint could not "lose" its registration. More excactly, that registration would expire based on a expiry value which is negotiated by the server and endpoint during the initial registration. An endpoint will generally send another Register Request prior to the expiration of its registration in order to renew that registration. If it does not recieve a response to its registration requests, it becomes "unregistered." Most endpoints will continue to attempt registration until service is restored.
I hope this clears up a few of your questions about SIP and Asterisk.