The official announcement of the YAHOO/MSN IM interoperability moved over the wire earlier today.
According to the conference call things like one single ID between both services will be all you need and yes, it will provide VoIP calls between both IM client’s users. Initially this is going to be PC to PC according to Yahoo VP Brad Garlinghouse, who said “they want to get that experience right.”
The blended IM service is expected to come available in the 2nd quarter of 2006.