I wanted to do this also. Here's what I did. Caveat: I'm new to WordPress and Thematic ... and I'm just getting started with PHP. In short, I'm no expert. I thought about working on the .php files, but felt I was a little over my head still. So I decided on this very simple solution for now:
Just type all your HTML right into the footer text field in the Thematic Options area on the back end of Wordpress. It seems you can type any kind of html in there. It all is placed inside the "siteinfo" div. I wanted a three-column footer, so I created three divs and put all the content I wanted inside them. (It's all static content, so this was easy, simple html).
Then just position the new divs you created in your child theme's style.css.
The way my footer lays out, I think the "siteinfo" div is kind of unnecessary. It's an extra layer that I don't need. What I originally wanted was another two divs at the same level as "site info". But just putting three new divs inside site info it works fine. It's not as clean as it could be, but someday when my php gets better, maybe I'll go back and look at it all again.