Say what you will about Microsoft's nonsensically named PST format—it’s the data format behind Outlook Personal Folders, according to Microsoft—but for hundreds of millions of people around the world, it's the engine that drives their most essential communications tool. In fact, at a recent Microsoft event, I was told that many of the company's customers didn't even realize that the tool they used was called Outlook. But they know all too well that it's the interface they live in all day long.

The ubiquity of Microsoft Outlook, and of the PST data format that drives it, makes this technology core to the software giant's popularity with business users. But you may be surprised to discover that almost 50 percent of Outlook users are actually consumers, or at least individuals. The amazing diversity of all of these users' needs will, of course, continue driving the product design of Office and Outlook for years to come. But the size of this audience also speaks to the need for the data format behind Outlook to be opened up to the outside world, much in the same way that the Open XML data formats, SMB network file sharing, and other core Microsoft technologies have been opened up in recent years.

It's obvious in hindsight. But Microsoft's announcement about its intentions to open up PST on Monday came as a surprise to me. According to Microsoft, previous interoperability schemes for PST, which were based around developer APIs, work fine in homogeneous environments where Outlook is always on the desktop. But this model is limited when true interoperability is required between disparate server and client systems, most of which don't natively support PST today.

So Microsoft will provide documentation so that any third-party vendor can recreate the PST format on other platforms, for free and without fear of recourse, and interoperate with PST files more seamlessly. The potential results of this transformation are far-reaching. For example, imagine Google email servers that interoperate directly with PST file stores on Windows PCs, Macs, or Linux boxes. Or third-party email solutions that can take an existing PST file and make the migration to a new solution easier than ever before.

In short, this decision will aid Microsoft's competitors, just as licensing the ActiveSync protocols behind Exchange Server do. But according to the software giant, it's all for the greater good.

"Designing our high volume products to enable data portability is a key commitment under our Interoperability Principles, which we announced in early 2008," Paul Lorimer, the group manager of Microsoft Office Interoperability wrote in a blog post announcing the decision. "We support this commitment through our product features, documented formats, and implementation of standards. The move to open up the portability of data in .pst files is another step in putting these principles in action."

This is a tremendous commitment to something that seems obvious in principle but must come under rigorous debate in practice. What it will require of Microsoft, of course, is a continuation of its steady progress in improving the core capabilities of Office and Outlook, all while preventing customers from adopting potentially less expensive alternatives that, soon, will be ever more compatible. That Microsoft is making such a move now also speaks highly of the company's confidence in future versions of Outlook and Office.

Speaking of which, Microsoft will soon release a public beta version of Office 2010, and that should give businesses a near-final look at the heavily improved and revamped Outlook 2010. I'm not personally much of a fan of Outlook, having converted to simpler web-based solutions years ago. But I'll take another look. Hundreds of millions of users can't be wrong.