I realize this answer is 4 years late, but the existing answers are misleading people.
The right way to do this is never __file__, or trying to walk through sys.path and search for yourself, etc. (unless you need to be backward compatible beyond 2.1).
It's the inspect module—in particular, getfile or getsourcefile.
Unless you want to learn and implement the rules (which are documented, but painful, for CPython 2.x, and not documented at all for other implementations, or 3.x) for mapping .pyc to .py files; dealing with .zip archives, eggs, and module packages; trying different ways to get the path to .so/.pyd files that don't support __file__; figuring out what Jython/IronPython/PyPy do; etc. In which case, go for it.
Meanwhile, every Python version's source from 2.0+ is available online at http://hg.python.org/cpython/file/X.Y/ (e.g., 2.7 or 3.3). So, once you discover that inspect.getfile(datetime) is a .so or .pyd file like /usr/local/lib/python2.7/lib-dynload/datetime.so, you can look it up inside the Modules directory. Strictly speaking, there's no way to be sure of which file defines which module, but nearly all of them are either foo.c or foomodule.c, so it shouldn't be hard to guess that datetimemodule.c is what you want.