Invalid Package declaration in script (Groovy)

I encountered the following error when using the Eclipse Grails Plugin:

Invalid Package declaration in script: C:\Documents and Settings\bparks\self\bookclub\grails-app\conf\spring\resources.groovy is not in a source folder matching the package declaration: resources.groovy

That error was showing up in the 'Problems' tab in Eclipse.
Easy answer was found in this post in the grails-user mailing list archives --

1. Go to Groovy project properties (in main project properties)
   
2. Checkbox "Disable Check Package Matches Source Directory"
3. Profit!
   

That fixed it, all is well.