Microsoft and Yahoo have followed through on a promise made nine months ago to enable interoperability between the companies' instant messaging networks. Beginning late Wednesday, customers will be able to beta test the functionality by registering their accounts.

The effort, announced in October, was expected to be complete last quarter, but Microsoft developers said they had to try a number of different technical methods to make it work. The Redmond company still considers the interoperability to be in beta, and adjustments will be made before the feature is available to all users.

Initially, the two companies have focused on enabling text messaging between Windows Live Messenger and Yahoo! Messenger with Voice. Voice capabilities across the networks will be supported at a later date, but cross-network video conferencing is not currently in the plans.

The agreement marks the first time major players in the highly-competitive IM industry have officially partnered up to enable cross-network communication. Interoperability has always been a hot topic among instant messaging providers, but had never yielded a compromise.

Yahoo and Microsoft together will command over 50 percent of the instant messaging market, with more than 350 million accounts in total. The pressure is now on AOL, which has long been the dominant force in instant messaging.

AOL and Google inked a deal to link their IM networks as part of an investment by the search giant in the struggling ISP, but no date for interoperability has been set.

In order to send messages from a Windows Live Messenger account to someone on Yahoo! Messenger and vice versa, users must download the latest builds of the software they want to use. Microsoft will make a new Messenger beta release available on ideas.live.com, along with a signup process to enable interoperability.

Likewise, Yahoo will host a beta signup on its Messenger with Voice Web site. Yahoo users must be running version 8 of the software, which is currently in beta.

The interoperability beta program will be limited, Microsoft said, but the companies are expected to roll out the feature to all customers within a few months.

"To add a Yahoo! contact in Windows Live Messenger, (after you've signed up for the beta) just click on the add a contact button. Then type their Yahoo! ID + @yahoo.com (example: example@yahoo.com)," explained Dan M.

"Your Yahoo! friend will get an invitation, and once they accept you are ready to chat, see when they’re online, nudge them, send emoticons, send offline IMs, and share your witty personal status messages...And though we are planning to add many cross-network features in the future, this should be more than enough to get you started."