Remember that agreement between Microsoft and Yahoo! to make their clients interoperable? It's happening.
It's been 8 short months since that deal was made, but it seems that the wheels for that to happen are already in play, and partially working right now. Yahoo's latest mac beta can right now send messages to MSN users. Yahoo said recently no that beta page: Coming soon: IM with friends that use Windows Liveâ„¢ (MSN) Messenger, but it is already doable with the current beta. All you have to do is log in to YM! with you Yahoo screename and log in to a windows PC through Windows Live Messenger (currently, you cannot use Windows Messemger or MSN messenger with this). Of specific note: You must log in with a @MSN.COM address. @HOTMAIL.COM accounts and other Passport accounts do NOT currently work.You are able to add @HOTMAIL.COM accounts but they do not send anything through, and other Passport accounts do not add at all.
As soon as you you do that, you will find that you can send a message to your MSN account through the Yahoo! software. Windows Live Messenger will not be able to respond back until it is enabled on their servers though.
For the step by step tutorial with pictures visit : Trillian Discussion Forums - YM! Mac messenger beta now able to send to Windows Live Messenger directly.