I still keep Photoshop and Illustrator around to prototype look and feel or specific design elements that can't be more readily achieved with CSS. Colors and type can at times be faster to experiment with in one of those tools. But I haven't sliced a PSD in years (except when developing another designer's work).
The other benefit is that your design can be readily kept in source control, in branches if need be, and if you have a good process in place and are thoughtful in your markup conventions, can be swapped between almost seamlessly.
Firebug/CDT are great for quickly tweaking a design...I imagine (hope?) the next wave of web design software would be much like those, except with an easier way to edit markup, work directly on files and integrate with source control. I suppose you can get some analogous with a browser reload plugin hooked to your text editor or filesystem.