Google's site wouldn't benefit much from switching to CSS layout. CSS layout improves download speed and can increase search engine rankings. But Google probably doesn't care much about their search engine rankings, and their pages have very little content, so speeding up download time isn't an issue.
If you want an example of a high traffic, modern website that doesn't use tables, look at ESPN.com. They've jumped on the CSS bandwagon, and even offer an alternate page to crappy browsers (NN4, IE4) explaining to them why they can't view ESPN.com with their browser, and offering links to upgrade or download a different browser. I recommend viewing their site in NN4. I wish all sites did this.