Not too early to go pure xhtml/css
The Creative Commons site is the biggest site (in terms of traffic and exposure) I've ever done in fully valid XHTML 1.0 and uses quite a few bits of CSS for layout.
I get all the webmaster email, and apart from a few bugs, typos, and general questions, I haven't received a single complaint about the layouts in Netscape 4 browsers (I hide all CSS from NN4 users). I thought we might be thinking too far ahead going with a full XHTML/CSS site, but so far it looks like the general public can handle standard markup.