From the press release entitled Yahoo! and Microsoft Bridge Global Instant Messaging Communities we learn

SUNNYVALE, Calif., and REDMOND, Wash. — July 12, 2006 — Yahoo! Inc. (Nasdaq: “YHOO”) and Microsoft Corp. (Nasdaq: “MSFT”) today will begin limited public beta testing of interoperability between their instant messaging (IM) services that enable users of Windows Live® Messenger, the next generation of MSN® Messenger, and Yahoo!® Messenger with Voice to connect with each other. This interoperability — the first of its kind between two distinct, global consumer IM providers — will form the world’s largest consumer IM community, approaching 350 million accounts.1

Consumers worldwide from Microsoft and Yahoo! will be able to take advantage of IM interoperability and join the limited public beta program. They will be among the first to exchange instant messages across the free services as well as see their friends’ online presence, view personal status messages, share select emoticons, view offline messages and add new contacts from either service at no cost.2 Yahoo! and Microsoft plan to make the interoperability between their respective IM services broadly available to consumers in the coming months.

The Windows Live Messenger team also has a blog post about this on their team blog entitled Talk to your Yahoo! friends from Windows Live Messenge which points out that Windows Live Messenger users can sign up to participate in the beta at Once accepted in the beta, Windows Live Messenger users can add people on the Yahoo! IM network to theor Windows Live Messenger buddy list simply by adding new contacts (i.e. add 'Yahoo ID' + to our IM contact list). Windows Live Messenger users don't need a Yahoo! account to talk to users of Yahoo! Messenger and vice versa. That is how it should be.

Where it gets even cooler is how we handle Windows Live Messenger users that utilize an "" email address as their Passport account Windows Live ID (e.g. yours truly). If you add such a user to your IM contact list, you get the following dialog

You then get two buddies added for that person, one buddy represents that contact on the Yahoo! IM network and the other is the same buddy on the Windows Live IM network. This is a lot different from what happens when Windows Live Messenger interops with a corporation that uses Microsoft Office Live Communication Server because people are forced to change their Passport account Windows Live ID to an address to resolve the ambiguity of using one email address on two IM networks. I much prefer the solution we use for Yahoo! IM interop.