New on MSDN: Introducing InfoPath Development. A good introduction to the object model and development environment.

There are plenty of neat things about InfoPath. A few which aren't mentioned in this overview:

• The taskpane - HTML, which can interact with your form (to guide users through form-filling, or give other context).
• The ability to attach event handlers to any part of the DOM, by adding <xsf:domEventHandler> declarations in the XSF...
• Ditto, for custom validation (although you get UI to do this, too)