You can build JBoss on OSX, or use the version that is included with Xcode Tools 1.1. Actually, it is no longer part of Xcode, but a separate download...
From the appserverdev_7k248_read_me.pdf file from the new, separate pkg (available at the ADC, http://connect.apple.com):
The Java Application Servers Developer Preview package was previously released as an optional install with the Xcode Tools 1.0 and 1.1 installer. It is now distributed as an independent package for installation. Changes in this package include delivery of JBoss 3.2.3, upgraded from version 3.2.2RC2, and Tomcat 4.1.29, upgraded from 4.1.24.
I added some notes on the Wiki about getting this to work (problem w/ the JAVA var in the default setup):