Since version 2.1 of the Maven Dependency Plugin, there is a dependency:get goal for this purpose. To make sure you are using the right version of the plugin, you'll need to use the "fully qualified name":
mvn org.apache.maven.plugins:maven-dependency-plugin:2.1:get \ -DrepoUrl=http://download.java.net/maven/2/ \ -Dartifact=robo-guice:robo-guice:0.4-SNAPSHOT