Responsive Base is a child theme built on the mobile first approach to responsive web design. It was created to act as a template or starting point for child theming with the Thematic Theme Framework and WordPress. It is perfect for beginners who want a head start with responsive themes for Thematic.
It is intended to work best with the latest version of Thematic old versions of Thematic (before 2.0).
This theme also plays nice with the Thematic HTML5 Plugin available at http://wordpress.org/plugins/thematic-html5/ which helps give you a very modern HTML5 to go along with your responsive theme.
UPDATE
This theme is getting old, however it still may be useful to some. There is a new version of Thematic coming (2.0) which changes enough things to make some of the code in the functions.php obsolete and the CSS could be better reworked to take advantage of using the new classes instead of the old ID's.
It will still work perfectly fine with 2.0 if you set it to use the old XHTML markup, since Thematic is backwards compatible. To do that, go into your WordPress Dashboard, in the menu Appearance > Theme Options, you will see a "Restore the legacy XHTML Doctype and xhtml compatible markup.", make sure this IS checked.
Personally, I never use this theme because it is no where near my current work flow, if you want to see what I use as my responsive base theme, check out http://scottnix.com/responsive-base-plus/ which is made for the 2.0 version of Thematic and all the new features it provides.
http://scottnix.com/themes/responsivebase/
http://scottnix.com/responsive-base-a-child-theme-for-thematic/
Anything to add? Improvements? Create an issue on GitHub.