As part of the initial accessibility work in Moodle 1.6 a table-less 'weekcss' course format was demonstrated. The significant point is that the linear view of Moodle's 3 columns with a table is, Blocks - Main content - Blocks. With 'weekscss' it's Blocks - Blocks - Main content - all blocks are together. (This order is a result of the stylesheet which applies margins to the central content and floats the left and right columns.)

http://alistapart.com/articles/workingwithothers, section "What do you mean, nav should come first?" suggests that blocks first can be preferable.
The existing 'Skip block' links will help, but a 'Skip navigation' link (Bug MDL-4994) would be better.

Can the other course formats be made table-less (discussed with Martin D)? - lams, scorm, social, topics, weeks. The OU uses a 'study calendar' format which uses similar CSS to the 'weekscss' format, so I consider it well proven.