Today the Mac BU of Microsoft released Messenger for Mac 7. The new version was announced back in November 2007 and adds audio/video support for corporate users only. According to Microsoft, it continues to work with the Windows Live team to bring full A/V functionality to all users.
Messenger for Mac 7 delivers new support for Mac OS X Bonjour instant networking technology, a new contact list search tool, and the ability to give contacts a nickname for easy recognition, even when their screen name changes. Corporate Mac users gain increased support for Office Communications Server 2007 making it easier to search a corporate address book from within Messenger as well as new support for Mac OS X Bonjour instant networking technology.
Corporate users can also participate in instant face-to-face meetings with co-workers using multi-party audio and video conferencing. Enhanced presence features allow co-workers can quickly see each other’s availability.
The Mac BU promised more details in "the coming months" for timing on A/V support for personal use and it appears that MSN still only interoperates with Yahoo! IM, which is unfortunate.
My poll from November 2007 indicated that 60+ percent of respondents use MSN for their IM needs – which seems high to me. Here's that poll again, in case you didn't get to vote the first time around.