The following steps work for a new app (Grails 1.1.1) to run without using a datasource:

grails create-app nodb
cd nodb
grails uninstall-plugin hibernate
rm grails-app/conf/DataSource.groovy
grails create-controller Foo
<add "render "hi bar" to the index closure of ./grails-app/controllers/FooController.groovy>
grails run-app
http://localhost:8080/nodb/foo - prints hi bar

For an existing app on at least version 1.1 (think that's when hibernate was made a plugin) you should be able to just uninstall-plugin and delete the DataSource.groovy file.