I have been working on this myself, basically as a 'pet project' for the sake of learning. However, I usually come to the conclusion that for producion sites (particularly high-concept designs) a pure CSS skin for DNN isn't practical right now (at least, for me).

Those reasons for me?

1 - Production Time:  Maybe I'm just still too 'green' with more advanced CSS layouts, but there are some things that I can either spend 3hrs fighting with Div/Span CSS layouts or just drop them in a table and go in less than 30minutes.  My current bane-of-choice... multiple columns, the floats and clears just never cooperate. When I finally do manage to wrestle them in, a module usually blows them out once it's inserted causing a howl of frustration and usually a feline getting punted across the room (no, not really). This leads to #2.

2 - Modules: Some modules are starting to move towards more CSS-friendly output but for the most part I still find them outputting to tables and blowing my Div/Span design out all over the place. My clients usually care about functionality first and rarely care much about the technology / code behind it. If it works well and ranks well, they're happy. This translates fairly simply to: if I create a skin that can't accept / work with the modules I need to provide the desired functionality I'm just causing myself more headaches and longer delays.

3 - Frameworks - I've tried YAML, Blueprint, DNNGrid, and several others. However they all seem a bit wonky and are still getting their sealegs... it also bothers me that alot of the push for pure CSS skins is to avoid code bloat and when I look at the rendered skin code for most framework, it's just as much of a mess but instead of Table, TR, TD elements, it's a mess of nested divs, spans, and so on. Add to this the requisite extra code for the DNN framework and I wind up feeling like I'm spinning my tires for naught.

4 - High-concept Designs - We all have those clients who want the next Oakley site but who don't have the budget. These are the ones I usually capitulate on the quickest when it comes to pure CSS, which leads back to #1 - time = money, if it has to be cheap, it has to be fast... and lately cheap rules the day, particularly with me losing at least one sale per month sniped by outsourcing firms. (however, I usually do enjoy hearing the horror stories that they come back with)

Sorry if I'm a bit all-over-the-place... end of the day and all.  Don't take what I say as the gospel, only as one developer's experience with trying to shoehorn in all the functionality of DNN and its third-party modules to a pure CSS skin while staying within budget and still trying to find time to sleep at night.

I'm open to any discussion or corrections.

Wells