Yea! To get the js-css-trickster-folks on board the first version works 'obviously' by splitting an image into the shadow and non-shadow parts offline. The result gets recombined in the browser by overlaying them and using some fancy blending modes.
Some micro-benchmarking should be able to show that the two shadows (jpg compressed 'obviously') plus css and js put together are smaller than a simple jpg compressed version of the input image...
Go! Go! Go! Before Hooli steals the idea... ;)