Just to be clear here:
Jython can certainly call Python libraries, because Jython is simply an implementation of the Python language for the Java platform. For the 2.5 release, our major focus has been compatibility with the 2.5 version of the Python language and specifically with the CPython reference implementation.
For previous releases, like Jython 2.2, I would suspect most Python 2.2 code would also run fine, it's just now we are trying to run larger, more ambitious code in Python as a community. Consider Django, Twisted, Pylons, or SymPy.
In my experience, most interesting libraries are written in pure Python. Often it's in pure Python to simplify support on Windows, or even for other implementations like ours or IronPython.
Now granted, there are libraries that don't run on Jython. These libraries are written to use the C extension API. The most significant ones I know of are PIL and NumPy. There are also others that optionally use C extensions to increase performance; we should run on them, but perhaps without the desired performance, until we have similar Java extensions in place.
Other libraries use ctypes, which has started to replace the use of the C extension API in most code. The good news is that we will almost certainly have ctypes support in our 2.5.1 release, using work from PyPy and JRuby.
Then there are libraries that rely on specific functionality, like os.fork or advanced signal handling. JRuby has had some progress on the former, and we share development on the underlying jna-posix library, regardless, these are Python libraries that would not be portable to non-Unix systems.