JBoss is a pure Java application server. So support for a 64-bit OS is really dependent on whether there is a Java JDK for that OS and whether that JDK can take advantage of the 64-bit architecture. JBoss 3.2.x requires a Java 2 JDK 1.3.x or higher. JBoss 4.0.x requires a Java 2 JDK 1.4.x or higher.
I know for a fact jboss runs on OS/400 (now iSeries)
which has been 64 bit for a number of years.