The comments to Simon's "Maintainability, a.k.a. the CSS elephant" have thrown up a couple of interesting items so far. Glazman pimps Cascades2 for Nvu, which allows you to view the rules that apply to an element and edit them in place.
There's also a comment from James McParlane about an experiment with CSS in XML. This is quite interesting - the nested nature of XML lends itself to describing a cascade, but I think the complexity of some advanced cascades could be a bit much for this idea...