I tried my hands at creating Grails plugin and I'm documenting my findings here.
1. First thing anyone who wants to develop plugin should join the grails-dev mailing list. There's too much hidden information which you just cannot get from the Plugin Developer Guide.
2. Of course you need to read other plugin codes.
3. I think the best way to get started is to simply get started with "grails create-plugin". Then simply package it and try to load it in your grails project. (See below for my kickass shitmore plugin)
Attempting to load  user defined plugins
Grails plug-in [shitmores] with version [0.1] loaded successfully
4. I did found out there is not easy way to test for plugin. The usual GrailsTestCase just won't do it properly. Well atleast not yet anyway. :(
5. The stable 0.4.1 release has problem with custom plugin, so you might as well be using the 0.5-SNAPSHOT. :(