There's a concept I've been meaning to write about for a while, but until I can fully articulate it, let me just let everyone in on the best reason (from a developer's standpoint) to move CSS-powered standards-compliant layouts: It's fun. It brings back the sense of discovery and curiosity and experimentation that's been missing in rote HTML layouts since we all figured out table hacks and spacer gifs five or six years ago.
There's plenty of business and bottom-line reasons to do it, of course, in addition to it simply being The Right Thing To Do. I've been having to review those at length for a work project, anyway. But the biggest benefit I can think of from the perspective of the dude at the keyboard making the pages is that it, simply, brings a little bit of that feeling of exploration back to making the web.
Ain't it fun?