A lot of people working with public facing web sites and SharePoint 2007 have discovered that the web part zone makes it virtually impossible to have a WCAG complient site, in other words to have a page layout which doesn't use HTML tables for layout purposes. The only possible way to change this is to make a web part zone adapter which renders the web part zone in a DIV based layout instead. The only downside to this approach is that this will break the javascript the zone uses to drag and drop web parts. This downside can be minimized if you only apply the adapter when the page is rendered to the end user, which means when the page is not rendered in edit mode. By doing this the javascript is only broken for the end user, so if the end user shouldn't mess around with web parts, then nothing is lost and the page will be compliant according to WCAG 2.0 (of course you would have to make sure the rest of the HTML is also compliant).

The below attachment is source code for the adapter with a little instruction in the readme file.