Source map work has begun here: http://github.com/clojure/clojurescript/tree/source-map. It promises a more complete and simpler solution to debugging ClojureScript directly in the browser.

This branch can encode and decode the VLQ64 format. It's based somewhat on the implementations present in Closure Compiler as well as the Mozilla version in JavaScript http://github.com/mozilla/source-map.

In this branch passing a :source-map compiler option will trigger the generation of the V3 source maps for the Closure compiled JavaScript. However this only maps the JavaScript generated by the ClojureScript compiler to the JavaScript produced by Closure. In order to be useful we need to read this source map and merge its contents with whatever information was produced by the ClojureScript analyzer. This would allow mapping all the way from a line in advanced compiled JavaScript to a line in the original Clojure source.

http://www.html5rocks.com/en/tutorials/developertools/sourcemaps/ has a good introduction to HTML 5 source maps.