This is something that i have been waiting for so long and finally it is a reality... I have found out today that the beta for the said service (cross-IM between YM and MSN) has been aready open. People willing to participate can now join this beta testing via this url: http://messenger.yahoo.com/partners_msn.php

From the yahoo website:

Yahoo! & Microsoft have joined forces

It's a great day for the world of instant messaging. Working hand-in-hand with our friends at Windows Live™ (MSN), we've connected our IM communities, making it even easier for you to stay in touch.

From the new Yahoo! Messenger with Voice, you can see when your Windows Live™ (MSN) Messenger contacts are online and send them instant messages. Keep all your friends in one place and IM with them anytime, right from Yahoo! Messenger. How cool is that?

Join the Beta