Add a :jar key to any cljsbuild options map and its cljs sources will be added to the jar. For this behavior, hooking must be enabled to leiningen.cljsbuild by putting :hooks [leiningen.cljsbuild] in your project.clj.
This is extremely convenient for doing library development in ClojureScript. This allows cljsbuild to compile in all 4 optimization levels at once, for easier testing, or to compile a test suite alongside the library code. Each build runs in parallel, with appropriate locking to ensure output isn't garbled. It seems to work well in both 'once' and 'auto' modes.