JBoss is 100% pure Java, so it should run on any Java platform. The recommended JVM version is jdk1.3. JBoss used to run with jdk1.2.2 with a little tuning, but this is not supported anymore.
For example, JBoss has been reported to run well on a variety of PCs with Windows NT / 2000 / Linux, on Sun/Solaris boxe, on MacOs X...
Be aware however that JBoss performance is very dependant on the underlying configuration. For example, informal tests show that on the same PC box, it can run twice as fast under Windows 2000 / Sun JVM than under Linux 2.2 / Sun JVM.
Linux users probably already know that linux does not support real threads. Under heavy load, JBoss will for example crash with 200 concurrent users under linux, whereas it can handle 1000 of them on the same box with Windows 2000. Of course, if you use Apache or Jetty in front of JBoss to handle the thread pooling, this will not be a problem.