Probably never. This is the exact same question people asked when Windows Media Player 10 was released, and after a few Microsoft completely dropped support for it. I know that's not good reasoning but the point is why would MS want to make better software for Mac? Aren't they supposed to be competing? Crude as that may be it does play a role in software development, I mean, we had messenger 4 and after a year we got messenger 5 with the brand new feature to now use display pictures!!!! Whoop de damn doo, while windows is seeing marvelous messenger updates. It's clear from this that MS is not the least bit interested in improving its messenger for mac. And even if they did, they have their hands full right now with Vista and Office 2007.
Take my advice, forget about msn, get aMSN or Mercury Messenger. Both are excellent clients and for the most part as good as msn. I mean they may not look as nice as msn but they can do voice and video conferencing, emoticon support, nudge support, custom emoticons, work fine with iSight. And the developers of these freewares are updating and making these better by the day, unlike MSN.