Arguably you can use webworkers (available on V8 and JSC, and thus Narwhal on those platforms) for concurrency problems. Webworkers enforce no shared state though, they use a message passing mechanism. Not the greatest solution.
Concurrency is important, but I think most server tasks don't require it. JavaScript doesn't need to be the 100% language to "win" it just needs to be the 80% language.
