When I opened Communicator I received a "Cannot Synchronize Address Book" message with additional detail of "Cannot synchronize with the corporate address book. The cause of the problem is unknown. Please contact your system administrator with this information."
I had this issue occur internally, i.e. on a local LAN/WAN and not from the Internet, and it turned out the IIS Virtual server for one of the address book nodes was mangled, i.e. returning unpredictable results. This was on a OCS Standard implementation.
When I browsed to the ABS node in IIS and clicked on Files, what I saw made sense. A list of address book files. When I opened up EXT\Files I saw the same thing. When I open up INT\Files, I saw garbage. I deleted the files directory and recreated it by pointing it to the same location that ABS\EXT\files was pointing to making sure the security was the same as the other two dirs. I repointed the abs\int\files dir to point to C:\Program Files\Microsoft Office Communications Server 2007\Web Components\Address Book Files\Files
Start, Run, IISreset /noforce and signing out and in to Communicator showed the address book was now accessible again.