It has been discusssed and suggested several times, however the MAC team are finally working on a huge update for MAC Messenger. Users as from the early months of 2009 are expected to have a new feature in their MAC Messenger which is considered to be the audio and video chat.
"The new Windows Live Messenger for Mac with AV support should be available as a beta sometime early next year."
Download MAC Messenger from here:
http://www.download.com/Microsoft-Messenger-for-Mac/3000-2150_4-10203624.html?tag=mncol;txt