Luke, there must be something wrong with this commit. If I do 'grails doc' with Grails 1.3.4.BUILD-SNAPSHOT, it fails since the import for grails.doc.DocEngine is missing in _GrailsDoc.groovy. After adding the import locally I get a classpath error, so I assume grails.doc.DocEngine is not on the classpath when 'grails doc' is executed. As a temporary workaround I commented out line 274 in _GrailsDoc.groovy.
I think this should be promoted to blocker, since it 'grails doc' fails, and consequently 'grails release-plugin' too.