are you going to make it some what easy to upgrade from the standard theme to the responsive one?
For the responsive child theme, upgrading is just installing and activating it. There is css in it that make the containers fluid, and have set breakpoints for certain widths to fit devices. Of course the whole theme with all it's shortcodes will not be responsive, so not all shortcodes can be used. Some shortcodes like bfthumbs (added fluid and responsive columns in 1.5), bfloop, columns will work though, but not the sliders. They have to be replaced with a responsive slider. The default blog, images and videos will be fluid though.
For upgrading to the full responsive theme, then there will be more differences, so a migration tool might have to be created (converting shortcodes/classes etc.).