ClojureScript is not yet self-hosting. That is, the compiler does not run in the browser, although quite a few people want it to, so it's being worked on a bit.

That said, I haven't found it to have any serious impact on my development experience. There is no compile-debug-refresh cycle because I can evaluate forms against the browser REPL remotely. Compilation occurs server side and the JavaScript code is eval-ed on the client.

More details here: http://blog.fogus.me/tag/clj-compilation/