Microsoft Messenger for Mac 7 does work with Office Communications Server but lacks some functionality compared to Communicator (featrues like Enterprise Voice, Desktop Sharing & other)
There is a new beta for the Mac but it does not work with Office Communications Server, so lets hope to see the next release with compatibility with and additional Office Communication Features

Microsoft Messenger for Mac 8 Beta Build 08.00.00.100305 Release Notes

Last updated 3/16/2010

The Messenger for Mac 8 Beta is for personal accounts only. To sign into your personal account you will need an ID on the Windows Live Network. The ID can be from any e-mail account ending in "@live.com", "@hotmail.com", "@msn.com", or other account that is registered with Windows Live.

Users with an account on a corporate network that uses Office Communications Server will not be able to use this beta.

More info:
http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=573c3149-ef09-4729-97ba-5200dd320ffe

Release Notes/ Known Issues in Messenger for Mac 8 Beta:
http://go.microsoft.com/fwlink/?LinkID=182393