[UPDATE: This library works with Mac OS X 10.6 Snow Leopard.]
In order to access a serial port in Java, you need the RXTX libraries compiled for your specific hardware. Java uses the Java Native Interface (JNI) to bridge between your platform-independent application code and the hardware-specific serial port drivers.
If you’ve tried this on an Intel Mac (perhaps to play with a Sun SPOT), you may be disappointed, since software keeps shipping from people that is either PowerPC only or isn’t compiled for 64-bit Intel. You’ve probably seen the UnsatisfiedLinkError message. Here you’ll find a librxtxSerial.jnilib file with support for both 32- and 64-bit PPC and Intel architectures, fitting the bill perfectly for both Java 5 and Java 6 on the Mac.
$ file librxtxSerial.jnilib librxtxSerial.jnilib: Mach-O universal binary with 4 architectures librxtxSerial.jnilib (for architecture x86_64): Mach-O 64-bit bundle x86_64 librxtxSerial.jnilib (for architecture i386): Mach-O bundle i386 librxtxSerial.jnilib (for architecture ppc7400): Mach-O bundle ppc librxtxSerial.jnilib (for architecture ppc64): Mach-O 64-bit bundle ppc64