I'm using JBuilder (7, I think) and I'm pretty sure the new version will be released for Mac OS X. Since it is a Jar package anyway, it should be easy to run it even if they don't package it for Mac OS X users.
You could also consider using NetBeans - although I've always felt that to be v.slow! But it's quite a nice env once you get used to it and there's tons of support available from the community.
I must say thought, that JB is my favorite, since its the only one where you can get a half decent Vi editor integrated into the IDE