Instant messaging and its ability to keep users within a portal’s network of sites will be the focal point as two rivals link IM systems to take on AOL’s market leader AIM.

Yahoo Messenger doesn’t talk to MSN Messenger, and neither talk to AOL’s AIM. Resistance to allowing other messaging clients to talk to members on different IM services has caused some grumbling among users with contacts who don’t use the same IM service.

That’s about to change, in a way that impacts millions of IM users, nearly half of the market, the Wall Street Journal noted this morning. Sometime today, Yahoo and Microsoft will announce their IM users will be able to talk to each other.

Text and voice messaging will be available as options between the two services. Yahoo users can chat with MSN users, and vice versa. The report points out the argument that the IM restrictions have never made sense; users with different phone companies can call any other phone number without a problem, so why not IM too?

A research firm, Radicati Group, quoted in the article said AOL has some 56 percent of the global instant messaging market, and a combined MSN-Yahoo would be near 44 percent. Google and other providers occupy the fractional share left over (the article said the numbers have been rounded up for comparison purposes).

Another factor, that of much-discussed Microsoft and AOL talks on a possible joint venture, could open IM interoperability to even more users. That move could be enough to break open the AIM barrier, if Yahoo proves willing to connect to the joint venture.

David Utter is a staff writer for Murdok covering technology and business. Email him here.