You can download JBuilder for OS X from Japan's Borland site, but it only works with Java up to 1.3.1, so if you've updated to Java 1.4.1, then you're out of luck... or so it seems... there are little hacks you can do to get JBuilder to run with Java 1.4.1 installed -- you'll just be hacking it to run with 1.3.1.
Well, I just looked on Japan's site and can't find the link I got it from. What I did to find it was just searched google.com for "Jbuilder mac os x download" and finally found it on Japan's Borland site. It's not listed, and a tech-call to Borland reveals that they don't produce JBuilder for Mac OS X anymore, contrary to what the website says.