We made a wordpress plugin for exactly this: ResponsivePics
Just pass the image id, image sizes and crops etc. in your template files and the plugin will resize them for you on the fly if the image is big enough.
In the 1.0.0-beta version we’re currently adding Action Scheduler for background processing the resizing of the images so your server won’t choke.
Wow, this is awesome! Have been using my own helper functions in conjunction with WP Retina 2x but this is so much cleaner… Thanks so much for sharing!!