Looks like Microsoft has released the PST format specification.

I don't normally like to link to MSDN, but I'll do it this once:
http://msdn.microsoft.com/en-us/library/ff385210.aspx

As usual with these documents, I recommend reading the PDF version rather than the HTML. Also, Firefox seems to handle MSDN a bit better than my (KDE 4.3.5) Konqueror.

If you were a mad-keen PIM hacker, and looking for a GSoC project, might be worth a look.

[Thanks to Tom Devey for the heads-up on this]