Hello, if you look at the Release Notes for Cisco Unified Video Advantage version 2.1 posted here, you will notice that Windows Vista is now supported in this version. I believe the expected release date for 2.1 is in the 1st quarter of 2008.
http://www.cisco.com/en/US/docs/voice_ip_comm/cipc/2_1/english/release/notes/ol113782.html
As far as I am aware, it is not planned to create a version of Video Advantage which runs on Mac OS X. The way you can have voice/video calls on OS X machines today is by using the Cisco Unified Personal Communicator application. This requires the use of an additional Cisco Unified Presence Server which interacts with your CallManager cluster, and the Presence functionality requires use of CallManager version 5.X or later. For more information on the Presence solution, check here:
http://www.cisco.com/en/US/products/ps6837/index.html