The great thing about Grails is that although on the surface it is simple and easy to develop with, underneath there is all the power of Spring & Hibernate.
As an example to perform authentication with Grails you could use action interceptors as described in the controllers documentation or alternatively if you want the power of declaritive security that a framework like Acegi for Spring offers then the underlying Spring configuration is there. In fact somebody has already done it and written a neat little walk through on how to go about configuring Acegi to work with Grails.
Power and simplicity, the perfect combination :-)
Codehaus Update: The distributations are available to download again, the images on the site are still broken, but the mailing lists are up! CVS is still down at the moment, but I've received assurances that it will be operational again soon.