There seems to be some cases of “Presence Unknown“, that are hard to troubleshoot. Normally we see to different types of this problem.
The first is Presence Unknown for Internal contact (users in the same Active Directory). This is typical related to internal server issues, or non existing (non UC enabled) users. To work with these issues, the best way is to use the server and client logfiles (Windows Event Log & Communicator logfiles)
In cases of Presence Unknown for External Contact, both federated and Public IM, the problems is often related to DNS name resolution issues. Again i can recommend to use the logfiles availible.
Troubleshooting with Office Communicator 2007 Logs
Public IM and Private domains:
Please post any comment below, also if you have relevant problems. I will do what i can to help.