I think the generator.__next__() documentation should say that it raises an exception if the generator is already executing:

http://docs.python.org/dev/reference/expressions.html#generator.__next__

I don't think this is currently mentioned anywhere in the section on yield expressions.

I think this is worth mentioning because this is different from the general situation for iterators, for example.  One consequence of this is that, unlike for iterators, using a bare generator in a multithreaded situation will always result in a critical section (since an iterator can be made to take care of its own locking, etc).