I know two real usages:

- the nose and py.test packages accept a generator function, as
described here:
http://codespeak.net/py/dist/test.html#generative-tests-yielding-more-tests
http://somethingaboutorange.com/mrl/projects/nose/#test-generators.
functions are collected with the help of the "inspect" module.

- the twisted framework use "inlineCallbacks": a function executes an
asynchronous operation and yields; execution is resumed when the
operation gets its results. "inlineCallbacks" is actually a a decorator.

Both cases make the difference between a generator function and a
regular function, even if it returns a generator. And they don't want to
execute the function to know it...