My mappings look like
static mappings = {
    "/"      (controller : "welcome",    action : "index")
    "/about" (controller : "welcome",    action : "about")
    "404"    (controller : "welcome",    action : "notFound")
}
This doesn't seem to catch 404 requests as I still get the default Tomcat 404 page. I have also tried the closure based method (below) with no success.
    "404" {
        controller = "welcome"
        action = "notFound"
    }
Versions:
versions: {
app.version: "0.1"
app.servlet.version: "2.4"
app.grails.version: "1.3.5"
plugins.tomcat: "1.3.5"
plugins.hibernate: "1.3.5"
}