For some time now I have been searching for a tableless design for phpBB. I did find one, called chairs. But so much had been changed and discarded that it wasn’t suitable for my needs. Brothercake, http://www.brothercake.com/ , had posted a possible solution on the Accessify Forums, but that involved changing code as well.
The designers and developers of WordPress are working on bbPress which will hopefully be as good as WordPress. It is still at the alpha stage of development at the moment, but it looks like an interesting project. Hopefully it will fill the void for an valid, accessible and standards compliant forum.