37Signals has an interesting post on why they skip Photoshop for web design. However there's an important caveat in their reasoning around point 5:
Photoshop is repeating yourself. Ok, so you've spent 3 days on a mockup in Photoshop. Now what? Now I have to make it all over again in HTML/CSS. Wasted time. Just build it in HTML/CSS and spend that extra time iterating, not rebuilding. If you're not fast enough in HTML/CSS, then spend the time learning how to create in HTML/CSS faster. It's time well spent.
The reality is that those folks who are able to go from a quick paper sketch to HTML/CSS are still a rare breed, they certainly don't come a dime a dozen even today. In the vast majority of cases, you'll have the creative design phase handled by people who master Photoshop way better than HTML/CSS (when they have a clue at web standards).
Sure, when you get the chance to work with people who are able to master both web design and web development, it's simply wonderful. I've tasted that and cannot agree more. But, hey, just take a look at 37Signals' own jobs board and tell me if you don't clearly see that dichotomy in professional competencies all over the place!
I'm not going to feel bad if I start with a tool that feels more comfortable to me during the creative process. Nor will I shame any other designer I work with for the same. Especially if other tools limit creativity. In any design medium (print, web, interior, industrial, fashion, even architectural design...), I would not be a true craftsman until I intimately understand how my design gets produced and the materials needed to do so. This most importantly includes the innovations and compromises that need to happen along the way because current production techniques can't recreate my design.
When starting any design, choose the tool (or set of tools) that's not only right for the job, but also right for you.
Douglas Bowman in Choosing the right tool, with a few other responses: