In Grails 2.2.4:
The "log" logger is injected into the application's main BootStrap.groovy and into the plugin's descriptor (e.g.: FooGrailsPlugin.groovy)
The logger in the app's BootStrap.groovy has a name like "grails.app.BootStrap" so by enabling the appending of the "grails.app" logger in the configuration will allow displaying the messages sent through this logger.
The logger in the plugin descriptors has no package prefix, and named exactly as the descriptor class but without the groovy extension. E.g.: "FooGrailsPlugin", so it is not so easy to enable the log messages by the default injected logger. It doesn't help if you add a package definition into the top of plugin descriptor, it will not be used in the composition of the name of the logger.
Naturally, you can manually define a logger in the plugin descriptor (using a package name according to your needs) like this:
private static final log = LogFactory.getLog("yourapp.foo.FooGrailsPlugin")
After this, you can enable the "yourapp.foo" logger in the application and you will see the messages sent through the plugin descriptor's manually defined logger.