Yes, we need to do this to be able to retire the legacy invoker transports.
Probably not. There were only two initial reasons for JSR-160 implementation:
1. JDK 1.4 did not have it built in, so would need to drop in an implementation.
2. Performance could potentially be better with custom implementation.
Since are so far behind on JBoss implementation, don't know that 1 applies anymore (since JBoss 5 will be jdk 1.5, which has JSR-160 built in). As for 2, don't know how good/bad Sun's implementation is in regards to performance. Since remote jmx calls is not part of core JBoss infrastructure, may not really be an issue (where as jboss jmx microkernel was dependent upon jmx, rolling our own implementation made sense for performance reasons).
I personally don't care either way, just don't have the time/resources to devote to it at the moment (and don't know when I would as has always been a low priority).
jmx-remoting.sar is already included in HEAD and would use the JDK 1.5 JSR-160 implementation by default. Could also include this for JBoss 4.2 if desired. At that point, is only a question of performance (assuming JDK 1.5 or higher is required for JBossAS 4.2+)