I agree with cbVision. CSS3 and HTML5 are awesome and cool, but until IE8 and older are out of the picture, the new goodies just aren't practical. Currently IE8 is the most used browser, and its market share isn't dropping terribly quickly. I suspect it'll be at least another 3-5 years before we can safely use CSS3 and HTML5.
aj_nsc is correct that we can use the new doctype, and with some JS help, we can use the other HTML5 tags as well. Though, there really isn't any practical benefit to doing so. Visual browsers don't care if you're styling a <section> or a <div>; Google has stated, I believe, that HTML5 tags don't affect your page rank; and the most used screen reader, Jaws, doesn't do anything special with the HTML5 tags.
Today, HTML5 is more marketing than reality.