To add to the above comments, but Rich Hickey goes into the relationship between ClojureScript and Google Closure tools in his intro of ClojureScript. http://blip.tv/clojure/rich-hickey-unveils-clojurescript-5399498
Basic gist is in the commentsin the comments:
ClojureScript is a compiler for Clojure that emits JavaScript ... and is design to emit ... code [that] is compatible with the advanced compilation mode of the Google Closure optimizing compiler.