Updated my own website to XHTML 1.0 Strict from HTML 4.0 Strict. Pretty painless. Although obviously the site is still pretty ugly, difficult to navigate and ultimately pointless.
Trying to do the same at work to a site I wrote 5 years ago is a bit more painful, but I expect I'll get there.
I used Zeldman's Designing With Web Standards book to learn about XHTML.
Personally, I think the book contains too much talk and not enough reference material, but it's not so much aimed as standards zealots like myself but other so-called "Web professionals" who write web-sites that only IE can parse. Still, due to the chatty, backgroundy-nature of the book it does explain some of the harder parts in great detail, where a reference manual might have just confused me.