The best tutorial I have seen is at http://stopdesign.com/present/2004/ddw-seattle/tables/ the background behind the presentation is at http://stopdesign.com/log/2005/07/27/still-throwing-tables.html.
www.adobe.com has a few good articles on building your first tableless website. Do a search on Google for tableless xhtml site:adobe.com.
http://www.htmldog.com/articles/suckerfish/ Follow the beginner to advanced links on the left.
http://www.csszengarden.com/ Hundreds of website designs all with the same HTML code. Only the CSS file changes on each page. Study the CSS on each page to learn each designer's CSS techniques. Look at the following page using IE and then FireFox http://www.csszengarden.com/?cssfile=062/062.css. The explanation of what is going on can be found at http://www.w3.org/2005/Talks/11-steven-css-advanced/.
http://tutorials.alsacreations.com/modeles/ Has some basic tutorials and sample CSS for common tableless layouts.
www.alistapart.com Start with http://www.alistapart.com/articles/ALAprimer. Some of the best write articles for this website.
Another option is www.xhtmlized.com which will convert any website into XHTML/CSS from a graphic mockup.
http://www.cssreboot.com/ Join other web developers throughout the world in a simultaneous launch of CSS website redesigns. The next one is Nov. 1st.
Always check for valid CSS at http://jigsaw.w3.org/css-validator/
Check for valid HTML at http://validator.w3.org/
These are some of the best resources I have found. There are also several good books on the subject. Just look on Amazon.com or your local library can do an inter-library loan on just about any book. There are several XHTML CSS books and several with tableless design in the title.
Email Me is you have another resource to add to this list.