As of Grails 2.4.0, if you specify
-Xlint:deprecation
as a command-line option, Grails ignores this option and places it in it's DefaultCommandLine's undeclaredOptions HashMap. Grails explicitly sets the compilerargs for javac to
-Xlint:-options
This sets "no further warnings about command line options are generated from this point on".
You can see this in the Grails code on line 285 in the GrailsProjectCompiler.groovy
This is why setting JAVA_OPTS="-Xlint:-deprecation" or GRAILS_OPTS doesn't work. See the unresolved Jira issue at http://jira.grails.org/browse/GRAILS-7670?jql=project%20%3D%20GRAILS%20AND%20text%20~%20javac.