I recently discovered an excellent solution to the "Footer moves way up the page problem", and it only requires one minor change to Thematic. I thought you all might like hearing about it.
Here is a link to the solution: http://www.cssstickyfooter.com/
And the only change I had to make in my Thematic child theme was in (as you might guess) footer.php I moved lines 27-29 (the closing of the wrapper div) up to become lines 7-9 (just above "thematic_above_footer()").
This worked great for me, I hope it helps you too.