I want to second the point about not skipping the photoshop/fireworks/illustrator/etc. step like 37 signals does. The reason they are able to do it is that they have already established a fixed set of agreed upon design patterns that appear in their apps.
If you skip directly to css and html, it's very difficult to think like a designer because you feel all the constraints of css and html. You'll think things like "oh, yes, I'll put a 5px border here and make it have 5px margin from the element above because it's easy" instead of "wow, this block would be interesting if it overlaid the top block".
In photoshop, it's easy to experiment visually by moving around blocks. In CSS and HTML, it's a pain. (Although, tools like firebug and web inspector have made certain things easier, not all of it is).
If you skip directly to css and html, it's very difficult to think like a designer because you feel all the constraints of css and html. You'll think things like "oh, yes, I'll put a 5px border here and make it have 5px margin from the element above because it's easy" instead of "wow, this block would be interesting if it overlaid the top block".
In photoshop, it's easy to experiment visually by moving around blocks. In CSS and HTML, it's a pain. (Although, tools like firebug and web inspector have made certain things easier, not all of it is).