Reminds me of inlinecallbacks (http://twistedmatrix.com/pipermail/twisted-python/2007-Febru...) and imvu.task (http://thespeedbump.livejournal.com/63798.html).

Their Deferred type looks like it's not thread-safe or able to handle reentrant invocations, which seems like it would limit the usefulness of this framework in the real world, since many asynchronous APIs require you to be able to handle being invoked from multiple threads.