I understand your general gist... but I find sketching the design by pencil and paper much faster than photoshop - obviously I don't get pixel perfect results, but the intricacies of CSS mean that it's difficult to match a photoshop design pixel-by-pixel anyway. I agree that it's much better to have a visual plan in place first, but photoshop would definitely slow me down, and I've designed dozens of web sites.
Pen and paper should be used as a tool to help in the thinking process, but it is not a replacement for real computer mockups.
Sketching doesn't give you a realistic sense of space and feel for web design. Very often you can sketch something that looks good on paper, but once you put it on the screen it just doesn't translate well to a pixel medium.
Sketching is great if you are working with a physical medium like brochures or posters though. Then it doesn't suffer from the same translation cost.
It depends a lot on where you're iterating. If you're iterating concepts and structure, sketches work perfectly. If you're iterating the size of a shadow, the exact color pallet, the exact shape of a logo then you'd better be in Photoshop.
For all 37s likes to talk about not using Photoshop they don't have pixel-perfect design at all and yet still had somebody slaving over Adobe products while putting those logos together.
Maybe the goal of working in Photoshop isn't to mimic the design pixel-by-pixel, but instead to "sketch."
Mocking things up in Photoshop can be really productive because everything you sketch (all of your background images, buttons, and art) can be used as an asset on your site with minimal effort.
