This Grails plugin uses yUML (http://www.yuml.me/) to draw class diagrams for your Grails Domain classes.
This project is now hosted at: http://grails.org/plugin/create-domain-uml
To get the plugin:
grails install-plugin create-domain-uml
To run the plugin:
grails create-domain-uml
This will create a file DomainUML.html in your project root which contains the diagrams.
Example (Using the domain from Grails In Action):