Not long ago, corporate wisdom had it that content and customers were best kept behind tall garden walls, but the recent announcement that Microsoft and Yahoo will open up their networks and allow their respective instant messaging users to talk with one another is yet more proof that those walls are tumbling down. While altruism may have played some part in the deal, it is likely that the two partnered in hopes of overtaking AOL with its 56% of the current market share. They’ll have a rough row to hoe. AOL’s software is ubiquitous and friendlier than either Yahoo or MSN.

Plus, could the hand-shake be too little too late? Upstarts like Cerulean Studios’ Trillian and Defaultware’s Proteus X for the Mac have been providing free software allowing individuals to chat across all three systems for a while now and they come with a friendly array of customizable features, among them video and SMS support for forwarding messages to your phone.

This is all well and good for the user who just wants to chat with their friends and colleagues across systems, but the real competitive edge may well turn out to be voice over IP (VoIP). The recent $2.6 billion purchase of VoIP company Skype (whose tagline reads “the whole world can talk for free”) by eBay was considered by many to be chancy, but signs are good that individuals will choose the much cheaper (free!) VoIP services over more traditional telephone providers whenever it’s possible and easy. The Microsoft/Yahoo partnership will make it very easy for users of those two systems.

The real story may lie in rumors of talks between Microsoft and AOL’s owner Time Warner to discuss more interoperability between those two chat and voice systems. There’s been little love lost between the two in the past but market pragmatics could force them to at least kiss for the cameras. Meanwhile, newcomer to the messenger game, Google, is also said to be in talks with AOL. It will be worth watching how this all plays out.