Both Yahoo and Microsoft had said previously that users would be able to communicate across platforms by second quarter of 2006. With the second quarter over, representatives from both companies now only say that the interoperable service will launch globally "very soon."

When both the companies announced interoperability late last year, they said the service would let users exchange messages, see buddy online presence status, share some emoticons and add buddies from either service to their lists. Left out of the list, however, were services that would support some widely-used features such as voice, photo sharing or video conferencing.

"We will continue to jointly innovate and explore the delivery of enhanced services to users, including voice, which we believe is an important component of IM interoperability," Microsoft said through its public relations company The Red Consultancy Ltd. There are technical issues that the companies will have to address to support voice interoperability, the company said.

MSN Messenger is likely to interoperate with other IM networks in the future, the company said. "We need to assure that the costs of interoperation are in line with the business benefits," it said.

IM users have long asked for interoperability between clients but IM service providers have resisted as they compete against each other.

Market observers say that the threat of competition from Google's IM service, launched in the middle of last year, may have driven Yahoo and Microsoft to team up in hope of combining their weight against the new entrant. At the end of last year, Google and AOL said they'd make their IM services interoperable.

The two camps, with Google and AOL on one side and Yahoo and Microsoft on the other, even support competing protocols that enable the interoperability. While Google supports the XMPP (Extensible Messaging and Presence Protocol) protocol, Microsoft and Yahoo connect their IM offerings using SIMPLE (Session Initiation Protocol for Instant Messaging and Presence Leveraging Extensions).

SIMPLE and XMPP are competing protocols, both part of standards-making processes in the Internet Engineering Task Force standards body.

The war of competing protocols is affecting third party companies as well, with Cerulean Studios' popular Trillian IM client software apparently becoming a victim. The service allows users to combine many IM services within a single user interface, and was part of Google Pack, a collection of free software that Google promotes. It was abruptly dumped from the suite in May, without any reason being assigned by Google.