JBossAS 5.0.0.GA will support Java SE 6. JBoss AS 5.0.0.beta4 both compiles and runs with JVM 6, though you still need to copy the web services related jars to the endorsed directory (there is a JIRA that indicates this will be fixed by GA, hopefully by CR1).
Other people have told me that they are using Java 6 with JBoss 4.2.2. I believe this is not supported, at least from what I can glean on this website. Is it?
If not, are there known issues documented somewhere? Or is just a case that the JBoss team is not going to test old versions of the app server with 1.6?
Is there a page somewhere that clearly lists JBoss app server versions and their highest supported JRE?
No, it is not supported. "Supported" means that if you run into a problem, and you are using JDK 6, and you are asking for a solution to the problem, the first thing you will be asked to do is try it on JDK 5. If the problem goes away, that is the fix.
The main problems with JDK 6 is that it supplies incompatible versiosn of the JAX-WS librariess. See the discusson at http://www.jboss.com/index.html?module=bb&op=viewtopic&t=102355&postdays=0&postorder=asc&start=10.
Instructions for running with JDK 6 are here: http://wiki.jboss.org/wiki/JBossWithJDK6
The supported version matrix is here: http://wiki.jboss.org/wiki/JBossVsJavaJDKVersionMatrix