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'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: