First proof I've got Magnolia fully integrated into Grails. It was a struggle but I felt it must be possible and here is the first screencast showing that it was possible.
UPDATE: Check this for a more complete screencast and links to plugin. http://www.kimmo.se/2012/02/maglev-02-released-magnolia-grails.html
Running Grails Controllers as Magnolia Templates, Grails Controllers as Magnolia Paragraphs. Render views with GSP and access Magnolia content at the same time, and much more that isn't shown in the screencast.
More to come...