Modify the build system to make sure the build is using Java 6, and nothing else. This prevents a ton of problems later on with database drivers, etc.

Put out a huge warning and disclaimer out into the console that:

MAKE SURE YOU ARE RUNNING JAVA 6.
IF YOU ARE NOT, YOU ARE VYING FOR A WORLD OF PAIN.
MAKE SURE ALL JARS SUCH AS DB DRIVERS ARE COMPATIBLE WITH JAVA 6.

etc.