The news was announced by Paul Lorimer, Group Manager at Microsoft Office Interoperability, on the Interoperability@Microsoft weblog. Microsoft says that data portability has become very important, especially now that all sorts of crucial information is stored in closed formats, and more specifically, in Outlook .pst files.
To meet the needs of customers and partners, Microsoft will release detailed technical documentation about the Outlook .pst format.
This documentation is still in its early stages, and the company is gathering feedback from partners and interested customers about the documentation, to ensure that it will be "clear and useful". A wild guess is that it might arrive alongside the release of Office 2010, but I'm not basing that on anything other than common sense.
The documentation will be released under the Open Specification Promise, meaning that anyone can implement the .pst format in any away, on any platform, using any tool, free from any fears of patents. You do not need to contact Microsoft in any way.
A good move by Microsoft, but long overdue. Better late than never, I suppose, and it will also benefit open source Outlook equivalents like Evolution.