Cisco Unified Communications Manager (aka CallManager) version 4.2, 5.1 and 6.1 (specific version/release below) is now certified for Direct SIP connectivity with Office Communications Server.
Direct SIP allows connectivity between the OCS mediation server and the Cisco IP-PBX without the need for a gateway, meaning that Office Communicator users can be enabled for Enterprise Voice (i.e. Office Communicator is a full soft-phone, not just click-to-call or RCC) without significant upgrades or hardware replacements.
More detail at http://technet.microsoft.com/en-us/office/bb735838.aspx
Qualification Level
Cisco
4.2[3]sr3a
5.1.3.3000-5 & 5.1.3.1000-12
6.1.1.3000-2
(requires update package for Communications Server 2007 Mediation Server: August 2008)
I have a few OCS related posts in the wings but I've been tied up with travel, work and home activities, so they are currently stuck in Draft mode. I'm still behind on many items I need to complete before I can even think of finishing off those articles