Microsoft and Yahoo! announced that they will make their instant-messaging programs work together, a partnership ostensibly designed to give the combined companies more heft to compete against AOL.

The partnership, which would allow users of the two services to exchange messages seamlessly, gives the companies almost as many users (combined) as AOL has on their own. AOL’s AIM, has over 50 million unique U.S., compared to about 27 million for MSN Messenger and 22 million for Yahoo’s Messenger.

The companies said that they expect the service to start by June 2006.

On the Microsoft Web site, there’s a picture of Brad Garlinghouse, Yahoo!’s vice president of Communications Products shaking hands with Blake Irving, corporate vice president of the MSN Communication Services and Member Platform.

Mr. Garlinghouse is slated to give a keynote address at the upcoming Internet Telephony Conference & EXPO on October 24th in Los Angeles at the LA Convention Center.

Of course, that got me thinking… With all of the buzz surrounding consumer VoIP offerings merging voice and IM capabilities (from the likes of Yahoo, AOL, Google…) one realizes that it’s not much of a stretch to add voice peering to the service announced today by MSN and Yahoo!

Suddenly you have 50 million people able to see each others’ presence, able to IM each other, and able to speak to each other utilizing VoIP. That’s just the Microsoft/Yahoo! camp. Now add 50 million AOL users, and let’s not forget about the 50 million+ Skype users plus the estimated 135 million registered eBay users.

That’s a lot of VoIP. Oh my.