From a blog posting by Paul Lorimer, Group Manager of Microsoft OfficeInteroperability:
Data portability has become an increasing need for our customers and partners as more information is stored and shared in digital formats. One scenario that has come up recently is how to further improve platform-independent access to email, calendar, contacts, and other data generated by Microsoft Outlook.
On desktops, this data is stored in Outlook Personal Folders, in a format called a .pst file. Developers can already access the data stored in the .pst file, using Messaging API (MAPI) and the Outlook Object Model—a rich set of connections to all of the data stored by Outlook and Exchange Server—but only if Outlook is installed on the desktop.
In order to facilitate interoperability and enable customers and vendors to access the data in .pst files on a variety of platforms, we will be releasing documentation for the .pst file format. This will allow developers to read, create, and interoperate with the data in .pst files in server and client scenarios using the programming language and platform of their choice. The technical documentation will detail how the data is stored, along with guidance for accessing that data from other software applications. It also will highlight the structure of the .pst file, provide details like how to navigate the folder hierarchy, and explain how to access the individual data objects and properties.
Read more here.