Well, if you look at the JMS spec (or is it on the JMS pages at sun), they quite clearly state that JMS is for intra-net,since a lot of the suff neede for it to work ouside of the corporate domain is missing. Here JAXM is the key (wich JBoss is still missing).
Agreed about JAXM - I was just about to reply to myself to comment on it but you got in first.
As to JMS, several commercial implementations support HTTP tunnelling. JMS and JAXM solve slightly different problems. If you're really good buddies with someone and share lots of code, you'll want JMS through a firewall, otherwise be circumspect and haul in ten-ton JAXM...