A part of the problem is that semantics are limited to the tag elements names. For example, a table when read to a screen reader must always be a table. In some instances it may be used for layout.
To address Dynamic HTML accessibility, we have a roadmap in place in the W3C to fix the problem. The intent is to allow the author a way to provide accessibility information into a web page to support the Accessibility API on the platform. I will be presenting information on this effort at the annual CSUN "Technology and Persons with Disabilities" March 14-19, 2005 for those who are interested.
Her is a link to the discussion overview: http://www.csun.edu/cod/conf/2005/proceedings/2524.htm