You're welcome! I tried coding this up at https://gist.github.com/darius/5287542 (untested). I see you have your own version now, which is longer but has a loop where I fell back to the tail call. (I also cut out a line in notify(), though you might prefer the old behavior of passing undefined for the results array on error, vs. the new one of an empty array. I guess I would prefer the old behavior on reflection.)

Some doc suggestions from my draft and I'll finally shut up:

    //  Can you call things after the first .await or .awaitAll?
    //  Whatever the answer, it should be documented.
    //  Maybe document the assumption that callbacks get invoked exactly once?
    //  (It'd be possible to ensure it's at most once.)
    //  Document assumption: the notified awaitAll function won't mess with its array argument
    //  (this matters if we're called again after).