lavalamp - thanks for the links. Honestly, I'd give my right arm to be able to do this the "correct" way, but it's simply not possible. Obviously I should have explained up front that this is my job, I'm working for a major client, and for the time being at least, this layout is just the way it is.
It's easy to be blinded by the CSS-fanatics who would have you believe that any use of tables for layout will bring plague and pestilence. In fact, tables are perfectly accessible as long as you are careful about how they are structured, the important issue being ensuring that the content flows in the correct order for user agents that won't be displaying the structure. Of course, you should still be marking up the content within the table correctly with paragraphs, headers etc. Tables for layout might be semantically incorrect, but there's nothing inherently inaccessible about a table, and while we're still dealing with varying browsers, tables for high level layout remain extremely useful, especially in the real commercial world.
Anyway, to answer your question! The idea behind the "table hack" is that you can have a left-hand navigation bar displayed in the right place, but within the code it's actually at the foot of the page for the benefit of screen readers, text browsers etc... an alternative to a "skip navigation" link.
I'd certainly recommend Joe Clark's book if you're interested in accessibility - lot's of useful advice - after all, how many of us have access to Jaws for testing? (oh, and lots of dispelling of myths too... )