Jason Rudolph has a great article showing how to use EJB3.0 Entity Beans step-by-step with the Grails framework.
Last time I tried with Grails 0.1 with Entity Beans there was some issues with package names and the generated scaffold code, so I hope that’s been resolved now. You can also define Groovy classes and mappings using Grails ORM (GORM) as an alternative approach.
Grails really is an awesome framework for rapid web app development, and I urge you to check it out if you haven’t done so already.