I liked developer Chris Coyier’s approach here; step back and examine how to serve up responsive imagery from a high level perspective. Chris touches briefly on all the proposed solutions, from creating new elements to client side Javascript.
On the not so cool side, Chris avoids taking a stance on which solution he’s siding with. I personally prefer a new image element or HTML5 based custom data attributes that the browser interprets to render the properly sized image. The other cross browser methods are admirable but ultimately make too many http requests for me to be comfortable with.