Python is the Borg of computer languages. Every language I've used previously to Python made it relatively difficult to interface to routines outside of the language. Python's object orientatied design and embeddable philosophy allows it to easily encapsulate code or data written in other languages.
JPython allows Java programs to transparently use Python classes and vice-versa. In effect this will allow design teams to introduce Python programmers to their existing Java development teams. JIT for the Python code is implemented making some Python programs run faster under JPython than the equivalent Java code! JPython is certified 100% Java so compatibility concerns have no basis.
Java is relatively weak at accessing the underlying operating system, on the server side Python allows Java programmers the ability to touch anything they want in the machine (with appropriate security clearances). On the client side, Python programs run within their own restricted environment running within Java's sandbox. Very safe.

Exciting stuff.