Awhile back I found web-app-theme and it looked like what I was looking for.
Upon closer inspection, it was RoR-centric, which wasn't so much what I was looking for.
Today I found YUI App Theme. I'm confident that it is "Just What I Was Looking For"!
Its a set of CSS designed as a starting point for web applications, and its built on top of YUI Grids.
I've only spent fifteen minutes with it and haven't made any special changes to the web application I happen to be working on (Regdel), but so far its made a drastic improvement. What a pleasant surprise! Another bonus is that its hosted on github.com, so I'm forking it and plan to make some modifications for my specific uses.
I have to watch out and make sure the YUI Javascript it employs doesn't clash with jQuery, which I make extensive use of.
I'm surprised there aren't more forks, only two I noticed, one of which makes some serious changes (like theme namespaces, though I'm not sure I'd want that):
UPDATE: I've branched the repository and made a couple of changes so far: