Since, oh, about forever, the only way programmers could work with the Outlook data store was via COM. It could certainly be done, but it wasn't as much fun as it might have been if Microsoft had, say, used an ordinary database for Outlook data.
That is beginning to change with the release of two new tools (both under the Apache open source license).
One is an Outlook PST Data Structure View tool. It's purpose is to give developers better insight into the data structures used in PST files. You can get it here: http://pstviewtool.codeplex.com/
The other is the PST File Format SDK. For now, this is a header-only, C++ release, but Microsoft is said to be readying the bits we'll need for complete read/write access. The PST File Format SDK is here: http://pstsdk.codeplex.com/
If your projects involve the need to access Outlook PST files, you'll definitely want to keep an eye on these.