You might have to upgrade to spring-security-ldap plugin to 2.0-RC2 in order to use Spring Security Core 2.0_RC2.
Reason: Notice the change in package name in latest version of security core plugin.
org.codehaus.groovy.grails.plugins.springsecurity.GrailsUserDetailsService
grails.plugin.springsecurity.userdetails.GrailsUserDetailsService