Since JBoss AS 5 we have offered two different distributions of the application server. For example, you can download jboss-5.1.0.GA.zip which is build with jdk5 and can run under both jdk5 and jdk6 runtimes (with a few changes), or go straight for jboss-5.1.0.GA-jdk6.zip which is build with jdk6 and can run only under a jdk6 runtime. (For details look in the release notes.)
Now realizing that (a) Sun's JDK5 is reaching its End-Of-Life in a couple of weeks (although for paying customers it is going to be supported for quite some time) and (b) around 70% of JBoss AS downloads are of the jdk6 type, we've decided it's probably time to move onto a jdk6 only distribution.
So, going forward, although the AS 5.2 codebase can still be build with jdk5, we will not be supporting it.
Does that present a problem for you? Let's us know before any final decisions are made.