Python and Java

[JKPeck]

Suppose you have an application written in Java, and you want to enable
other applications or processes written in Python to communicate with
it, i.e., to use Python as a scripting language for the application.
On Windows you could do this with COM and various addons such as
J-Integra and Mark Hammond's libraries.

How would you do this if you want a mechanism that is portable across
Windows, Linux, Mac, and Unix?

Any ideas?  Jython would be a natural candidate, but it is stuck at
Python 2.1 and seems to have an uncertain future.

Thanks in advance.