37signals had an interesting post earlier this month about how they skip Photoshop and jump straight to html/css from a rough sketch. I agree with this philosophy in principle. Once you start cutting the material that you'll ultimately deliver your product in, your working on something real. Real has dates, and quality, and tangible progress. And once your in the html, tools like Protonotes, CrazyEgg, and Google Website Optimizer can make your prototype that much more.