I posted elsewhere that I now own an iMac and am doing most of my Java development on it. The big problem with Java on Mac is that the JDK is pretty out of date. The official version that comes with Mac OS X 10.4 Tiger is 1.5.0_07. This version seems buggy, as the JVM seems to hang when running test cases for one of my projects. I was getting quite frustrated, and in the end decided to run my test cases under OpenSolaris, which I installed as a VM using VMWare Fusion.
Fortunately, Apple has recently made available a pre-release version 1.5.0_13 from its Developer Connection web site. You need to be registered to be able to download this release. The good news is that it fixes the bug that was causing my tests to hang the JVM. I think it has something to do with the java.util.concurrency classes. Ofcourse, if you don't use these, you would not have faced the problems I was facing.
The long gap between the Java releases on Mac seems unacceptable, especially since the JDK is so buggy. The other problem is the lack of support from major vendors for the Mac platform. I would have liked to install and use IBM WebSphere and Rational products on my Mac, but it isn't amongst the supported platforms. Linux has much better support now-a-days.