Just a quick post to mention that Processing Beta 101 is out and it includes PDF support. There are still some issues, but it's good news. I whipped up a few test sketches (nothing worth sharing) and the system works great. It's really nice to be able to switch between screen & print in the same tool. Here's Ben Fry's post giving some details on how to build a simple PDF illustration.
For a current project I've been looking at Scriptographer (which is also a cool website design, by the way). I've also been thinking of ressurecting some old Objective-C code and building a soup-to-nuts PDF constructor for this project. That obviously would be a lot of work. I also have some AppleScript code hanging around somewhere that would let me populate an InDesign layout with vectors and images generated from within Objective-C. But if I could somehow figure out how to get my project working with one of the Processing libraries or this new PDF support I would definitely prefer going the Processing route.
Whatever the case, Processing just keeps getting better and better.