Java SE 6 (64 bit only) is now on OS X and that is a good thing. As I understand it since Eclipse is still Carbon and thus 32 bit, it cannot be used for 1.6 on Leopard, only 1.5.
Does anyone know if NetBeans 6.x can be used with Java SE 6 on Leopard utilizing its JVM?
Yes, you should be able to.
A number of blogs have reported running Netbeans on 1.6 as well as the the problems they had with earlier versions of NB. The NB issue tracker also has a number of bugs that have been fixed that affected 1.6 on Mac OS.
If you have trouble getting it to run, you might also try the Netbeans forum.