FYI: I just updated the Plugin Extensions page to include the new plugin dependency and classloader changes introduced a couple of weeks ago.
This is the main page:
http://jopr.org/confluence/display/RHQ/Writing+Custom+Plugins
but the bulk of the new changes are found in its child page "Plugin Extensions":
http://jopr.org/confluence/display/RHQ/AMPS-Plugin+Extensions
Feel free to read it and give me comments/suggestions. These pages are only relevent and helpful to people writing their own plugins - if you are not a plugin developer, none of this will make any sense and you are free to ignore this :)