I have had success with a table-less site. It does take heavy customization of a lot of scriptlets and ConLib controls, but it will work. The scriptlet parts are pretty straightforward since its just messing with HTML. Editing ConLib controls are a bit more complicated, but I've worked out a process to convert TABLEd controls into DIVed controls. First step, open up the control's .ascx file. Use the find & replace feature of your favorite text editor to change all <table, <tr, <td, <th, </table, </tr, </td,</th, to <div and </div, respectively. Notice there are no closing tags ('>'), you don't want to skip over or overwrite any attributes the table tags may have had.
Once you have your DIV soup, you can start adding in your own CSS classes, deleting redundant DIVs, and generally making the control your own. DO NOT change any id attributes of any table (now div) tags; the code-behind files rely on these ids to do their fancy magics. Technically, there are some you can change, but make sure you know what you are doing first. If you need to move around ASP.NET tags, make sure you move the whole block. (<asp:xx>..</asp:xx>). Lastly, remember to save it in the custom folder!
You can look at
http://www.barknpurr.com to see a DIV based theme that was professionally designed and I integrated. It has taken me a few days to integrate his DIV based layout into a new AC installation. It takes maybe an hour to set up a page worth's of scriptlets, and the rest of the time is wrestling with ConLib.
I hope this helps someone. It is a lot of work, but I do believe it is worth it.