mio is doing pretty well as a most-popular event system and reactor. I wouldn't be surprised if it's blessed in the future, but at the moment the community seems to be there.

I'm also sort of hopeful that higher-kinded types can do something about writing code that can run on any such system. If you can have some functions on an abstract Promise<T> type, like fn then(Promise<T>, fn(T) -> Promise<U>) -> Promise<U>, it doesn't super matter which backend you're using. (As I understand it, JS is already here, because they can duck-type their promise spec.)