This post is a reply to @EmilEifrem's twition.
Neo4J could be a nice extension as a Grails' persistence backend, similar to a DB, space or LDAP. The problem today is that GORM is tightly coupled to Hibernate. Looks like @GraemeRocher is in the works of extracting Hibernate as a plugin, from which one can use other persitence plugins. Absolutely looking forward to that. (Graeme, please document what defines the folders which contain domain classes, controllers and views, so they can be changed!).
@Nawroth recommended using Neo4J through a JCR layer, though theres no implementation for Grails or Neo4J. The motivation for this is to be able to use the nice features such as MyPersonNode.find(), trinity.save() ... Recommend watching this clip 4 intro to Grails.
What can be done today is just using the Neo4J jar in Grails as it is. But the magic of GORM that makes Grails so different, compared to regular Java won't exist...