Pretty cool to make it transparent. Considering a lot of these have the model of a compiled language, this makes it much easier for a designer to work with. The only real problem I have is that it was made pretty heavyweight with the framework they included.
One really cool feature would be automatic CSS sprite generation. Add a "mixin" background-sprite, read in the image, suck up all the others as sprites and generate the final CSS and image file.
Any CSS preprocessor is basically transparent in its native language (in this case, PHP). The only time systems like Sass need a compile step is when they're being used with languages other than those they're written in, or with static CSS. Scaffold isn't transparent in these cases, since it simply doesn't support them.
One really cool feature would be automatic CSS sprite generation. Add a "mixin" background-sprite, read in the image, suck up all the others as sprites and generate the final CSS and image file.