Ron, I believe the phrase they're using is "not yet fully Leopard compliant". They have discovered some minor bugs that only occur in rare but repeatable instances and the programmer is a perfectionist so he's reluctant to fully sign off on it until he can work all the kinks out. Check out
their website blog for more details. Chances are you'll probably be able to boot from it and everything's fine. Just to be safe though, when a fully compliant SuperDuper comes out I'd re-clone your drive.