I use Dreamweaver in theme development, and I am curious as to what an Extension would add to the mix.
The single biggest issue in theming is organization. The biggest challenge is balancing flexibility and functionality. I can see that a tool that would let a designer declare regions in a template, place them visually, generating valid CSS, and see the results as they went would be useful. Unfortunately, while Dreamweaver can make it easier to work with the cascade, it is not so hot on the display side as its default behaviors (what is displays when a parameter is undeclared) are quite different from those of popular browsers.
Cary Gordon The Cherry Hill Company http://www.chillco.com