Last year Microsoft said that they would publish the format specifications for personal store files (pst) – formats used to store data from Outlook products, including email, contacts and diary appointments. You can find them at MSDN here: http://msdn.microsoft.com/en-us/library/ff387869.aspx. There is also format information for other Office products: http://msdn.microsoft.com/en-us/library/cc313118.aspx.
Related sites of interest include Microsoft’s interoperability pages, such as their ‘open specification promise‘ and Microsoft’s interoperability blog.
-Susan Thomas