If you are wondering, here are some facts.
There are two types of PBX integration. Enterprise Voice and Remote Call Control.
1) Remote Call Control NATIVE integration with Cisco is available without a IP Media gateway today with CUPS and CUCM 6.x. This can successfully set up.
2) Enterprise Voice functionality is currently only available today via a 3rd party IP Media gateway. It was expected that Cisco will support native SIP integration to OCS by Q2/Q3 2007. This will definitely require latest version at the time.
Microsoft like many other telephony vendors use their own SIP extensions that are not included in IETF RFC specs.
An one example, since both OCS and CM would in theory have call-control capabilities in an Enterprise Voice deployment, Cisco needs to adopt the MS specifications for Dual-forking to prevent looping.