Most recently, I experimented with using Axure to produce both product AND design specifications. I'm not sure if anybody has tried this, but it actually works quite brilliantly!
Although certainly not perfect, I found that it was a very effective way to create specifications for styles, spacing and sizing. It was surprisingly easy and accurate, using Axure's tools to create the measurements. The goal was not to be pixel perfect, but to convey the intended dimensions and styles to the presentation developers. With that in mind, it was perfect.
The func specs were added via image maps on top of the UI jpgs. I added additional fields for design specs. And then used red text, markers and dotted lines for spacing/sizing/bgcolor guidelines. Finally, I found, that if I kept all the visual markers in a dynamic panel, the prototype can double as a virtual or online pinup. You can even create a button that shows/hide the markers.
Overall, there are obvious benefits to using Axure for more than just UX and FA needs. With it's easy-to-use collaboration tools, a designer or art director can easily jump in to add their design definitions. Developers will no longer need to look in multiple areas for design, ux and functional directions.