I recently finished up the first cut of agent auto-update. I wrote up some documentation on it here:
Based on the work I did for this, I had to add remote configurability of the agent via the Jopr GUI (essentially, eating our own dogfood - I use Jopr to configure Jopr :) Information on this can be found on this page here, it talks about the agent plugin and the capabilities it introduces: http://support.rhq-project.org/display/RHQ/UsingTheAgentPlugin
Once this gets released, I'll probably get this content copied to the proper location in wherever our documentation is going to go.
The code for this is currently found in trunk and the feature can be tried today from a trunk build. I've done most of my testing on Windows so far (with a small amount of testing on Fedora) - therefore, if anyone is out there that tries to auto-update their agents which happen to be running on a UNIX box, please let me know how it goes :}
The design document page that I worked off of is here, for those interested in some of the implementation details: http://support.rhq-project.org/display/RHQ/Design-AgentAutoUpdate