Unfortunately Borland doesn't offer JBuilder officially for Macosx. However since it's written in Java it should run there, too?
It does and Im running the latest version (JBX) just fine :)
One hiccup is that they use InstallAnywhere and there is an incompatibility between the InstallAnywhere launcher and the developer JDK1.4.2 but you can run it manually with your JDK1.3.1. If anyone needs instructions, post and ill dope it out again. (Its the eay you run all InstallAnywhere stuff, basically you invoke one class in the install.jar and it does the rest.)
And yes, I love JBuilder :)