Users of MSN and Windows Messenger programs will be able to communicate with users of Yahoo Messenger, and vice versa, the companies said. The deal extends to presence awareness, text chat, emoticons and PC-to-PC voice.

With a combined base of 275 million accounts, the companies claim the network tie-up, scheduled for next spring, will create the largest IM system in the world, overtaking AOL’s AIM and ICQ networks.

The deal brings the long-sought dream of interoperable IM a little closer. For years, proponents of interoperability have pushed for IM to abandon its proprietary roots the same way email did well over a decade ago.

The problem has been lack of impetus. Consumer demand was high, but not high enough to push the three big IM firms to interconnect. There have been security and stability issues to consider, and business models were fuzzy.

Previously, Microsoft has offered its Live Communications Server 2005 enterprise customers the ability to connect to the AIM and Yahoo networks, but the company paid those networks for the privilege of connectivity.

In the consumer space, where IM is mainly ad-supported, it’s less clear what model could be employed. If it follows other models of Internet connectivity, it could be settlement-free peering, or transit based on traffic ratios.

MSN says it has 180 million Messenger users, so looks to be the larger network. However, it also has the most to gain in the enterprise space, where it is trying to make LCS 2005 as staple as Exchange.

The deal is forward-thinking, and could be interpreted as a voice-over-IP (VoIP) play, if one assumes that the IM networks and presence systems will continue to evolve as a platform for PC-to-PC and ultimately PC-to-phone VoIP.

With this in mind, the deal seems to take some of the initiative from mutual rival Google, which, if the speculation and hyperbole turns out to be true, has lofty ambitions in communications. Google released Google Talk this summer, a bare-bones IM client with VoIP functionality, and promised server-side interconnect with IM networks from rivals EarthLink and SIPphone.

While it’s too early to tell how successful Google Talk will be, the search company does have a large and loyal following and an increasingly powerful brand. And its IM software runs a different protocol to the one favored by Microsoft and Yahoo.

Microsoft has thrown its weight behind SIP, the session initiation protocol, and SIMPLE, a set of SIP extensions designed to handle presence information. These protocols are supported in its consumer and enterprise products. Google, however, built Google Talk on top of XMPP, an alternative protocol designed in and backed by the Jabber open source community.

It doesn’t look like a standards war is about to break out, however. EarthLink and SIPphone both use SIP too, and they plan to interconnect with Google Talk using SIP-to-Jabber gateways.

Furthermore, Google has stated that it is protocol-agnostic on this count. Indeed, the company’s director of product management, Georges Harik, told Computer Business Review in August that the company is as ready to support SIMPLE as it is to support XMPP.