From Microsoft:
As more and more information is stored and shared in digital formats, the ability for people to reuse their data across various applications and platforms has become increasing important. As part of an ongoing effort to enable this kind of data portability, Microsoft is announcing that it will be releasing documentation for the .pst file format – the format in which data is stored in Microsoft Outlook Personal Folders.
Providing access to the documentation will facilitate interoperability, enabling customers and vendors to access their data in .pst files across a variety of platforms. This is important to organizations that need exchange key corporate data in and out of Outlook, upload to the cloud, or comply with corporate governance policies.
When it is complete, the documentation will be released under our Open Specification Promise, which will allow anyone to implement the .pst file format on any platform and in any tool, without concerns about patents, and without the need to contact Microsoft in any way.
Paul Lorimer, the Group Manager of Microsoft Office Interoperability has posted additional details on the Interoperability @ Microsoft blog.