My application still works in JDK 6 but for development I used JDK 5 and specified JDK 5 in version.txt
so now Sun will fail me
Throughout this exercise, you must use exclusively the Java 2 platform. 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a production (not development) version of the Sun Microsystems' Java 2 platform and that platform must not have been superseded by a new production version for more than 18 months by the time you make your submission.
If you use JDK6 for development and testing then you should say JDK6 in your version.txt file. Of course JDK5 is also ok but if you used any JDK6 methods (especially in Swing) then running in JDK5 will not work.
Personally I don't know if JDK5 has obsoleted or not. So I recommend using JDK6. You may want to check Sun's JDK end of life page if JDK5 is outdated or not. Ok just checked
J2SE 5.0 is in its Java Technology End of Life (EOL) transition period. The EOL transition period began April 8th, 2008 and will complete October 30th, 2009, when J2SE 5.0 will have reached its End of Service Life (EOSL).