Reportedly MSN Messenger for Mac will not be updated, it will be replaced with a brand new version of Messenger for Mac OS X.
LiveSide Blog reports that in a presentation to the Georgia Institute of Technology's IEEE Student Branch yesterday, Microsoft employee and Georgia Tech (edit: graduate former student - Jenks went to work for MS before graduating) Andrew Jenks had some surprises in store.
MS will no longer update the MSN Messenger for Mac, but they are going to release a brand new client for Mac OS X that is according to him "Really, really cool and awesome" but he would not provide anymore details due to his NDA.