hi Eddy,
Yes, JDK 6 is allowed.
See instructions.html:
You may develop your code using any implementation of the
Java 2 platform, but the submission that you return must have been tested and shown to work under a production (not development) version of the Sun Microsystems' Java 2 platform and that platform must not have been superseded by a new production version for more than 18 months by the time you make your submission.
Herman