Twisted (a python asynchronous network programming framework) does something similar, but inverted. Twisted functions return a Deferred object without blocking. A Deferred is a promise of a value, so rather than passing the callback into the function, you register a callback with the deferred, and the function promises to call it later.

This type of thing has a few nice properties -- you can easily register more than one callback, as well as using them as a pipeline (kind-of like monads in haskell), which are awkward to implement with callbacks.

As an aside, in python you can trick generators into turning synchronous-looking code into asynchronous, which can actually make things look a lot neater if done properly -- i'll provide code if anyone's interested.