You can now sign up for the Yahoo! interoperability. Once you agree to the service agreement, you will be presented with a page saying to sign out and sign in again to activate the new functionality.
Windows Live Messenger 8.0.792 is required to use it, prior builds have been blocked from signing in, namely 8.0.0787. Your Yahoo! Messenger contacts will also need the latest beta version:
Does this work with all my Yahoo! contacts?
Yes. As long as your friends have the beta version of Yahoo! Messenger with Voice, you’re set. When you add them to your contact list, they’ll get all the information they need to gear up and connect with you.
Mario has posted his initial findings with the related changes to the new MSNP14 protocol.
As I have experienced some difficulty with signing up from the WLM side, I found this link that worked for me, and might work for others too -=>
http://ideas.live.com/signup.aspx?versionId=7adb59de-a857-45ba-81cc-685ee3e858fe