While interoperability in itself is great stuff, as a consumer of the Yahoo product, I can't say I am too excited about the MSN and YM hook up.
You like one or the other and if a lot of the people you want to instant message with use IM-ware what you don't you either pick a new crowd to hang out and chat with or switch your IM software - depending on what you are less passionate about.
In either event the coming together of Microsoft and Yahoo over a problem so trivial does not bode well for the end-user. I would hate to see YM loose any of its character and worse acquire a pronounced MS vibe. That would be such a shame.
If in time all IM software turned interoperable, that wouldn't be such a bad thing. Push a product into the market to best the competition, don't spend any time to develop standards within the industry. Realize a few years later that your product and the bested competitor's product could both benefit form talking to each other.
Return to the blackboard to start over. In the trenches, some people regurgitate code over and over and get paid for it thanks to corporate myopia. My friend R calls it - "job security". He claims he has kept jobs as long as he has because the powers that be don't see the big picture like they are supposed to.