From an e-mail exchange last night:
I think there’s an important relationship here: if content is the focus (as it should be), usability means structuring that content appropriately.
Structuring that content appropriately means designing it properly, but you can’t design until the content is in front of you.
Once you’ve designed it, it’s easy to look at the comp and figure out how to break down the content into structured markup (or, it is for me these days anyway — I build XHTML before I touch CSS). And then once the markup is done, you style it with CSS.
Really, in the end, CSS is the last step in the chain, and the least important. BUT. You have to consider it as early as the structuring.
If you can’t build it, well then you can’t design it. So even though CSS is the least important step, it’s still absolutely critical to the design process as a whole.
It’s the designer’s job to know the strengths and limitations of the technology, and how to make best use of it. That is why CSS is important to the designer; no more, no less.