I'm working on my first plugin for IDEA, and I'm documenting my learning experience in a "tutorial" for writing plugins.
It's very, very incomplete at this point (as my plugin isn't done yet!),but it's not a bad first step for someone starting out, and will hopefully soften the learning curve somewhat.
You can find it at: http://www.nurflugel.com/idea/Writing_plugins_for_IDEA.html
Comments are welcome.