Rich,
Because responsive design inherently only uses one html file, what you're describing isn't so simple. Obviously, you only want to include 1 GA tracking code, but you COULD include some logic that fires a different one on page load depending on detected screen width. I think your best bet is to just use the standard GA code in your HTML, then include some JS that detects which of the layouts are displayed on load, and sends it in a custom variable. Alternatively, GA, by default, will capture screen size as a data point, so, as long as your responsiveness is working properly, you can just create segments in GA based on those and view site behavior accordingly.
Mike