I wanted to draw attention to this article.
I agree with the original article's lamentation - why can't Adobe just stick to producing simple bloody documents?!? They used to do this so well. I swear to god - their offices must be lined with developers and architects just smoking crack and talking about what awesome features they'd like to embed in their products without any regard for other design or functional considerations! (ok, sorry for the vitriol - I'll get off my soapbox now).
Checking out the talks from CanSecWest this year I dearly wish I had the time to start doing some serious fuzzing with Adobe Acrobat Reader and playing around with the PDF format . I think this is an exciting space to be getting your hands dirty in.