Today I stumbled about a page that gives a pretty cool introduction in how to setup grails 1.1 in eclipse:
http://www.objectpartners.com/2009/05/27/eclipse-setup-for-grails-11-development/
The author also explains how to get rid of compiler errors that are due to the acegi plugin. This helped to erase almost all of them, for the rest I think I should check whether the grails path is set correctly.