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:
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]