When you want to develop a plugin for RHQ,
you can of course follow my plugin dev series. But in most cases you probably want to know more.
Of course, as RHQ is open source, you can just look at the source. Most of the time it is more convenient though to just browse api docs.
JavaDoc for plugin development is now available - check it out. The docs are for RHQ version 1.0.1.
If you have questions, join us in #rhq on irc.freenode.net or post in the RHQ forums.