Check this out
Yahoo Messenger interoperating with Live Messenger. A friend and I tried to get this work about a month or so back. Biggest trouble we had was being able to add to the IM list. I mean in Live, if I were to add myfriend@yahoo.com wouldnt it just think that that was just a passport id?
But its good to note that others have had success. Should revisit and give it a try.
Technorati : im, interoperability, msn, yahoo