I switched to a SNAPSHOT version of a plugin I use, and declared the corresponding snapshot repository in the POM like this:
<project>
<!-- ... -->
<build>
<pluginManagement>
<plugins>
<plugin>
<groupId>org.apache.openjpa</groupId>
<artifactId>openjpa-maven-plugin</artifactId>
<version>2.3.0-SNAPSHOT</version>
</plugin>
</plugins>
</pluginManagement>
</build>
<repositories>
<repository>
<id>apache.snapshots</id>
<name>Apache Snapshot Repository</name>
<url>http://repository.apache.org/snapshots</url>
<releases>
<enabled>false</enabled>
</releases>
</repository>
</repositories>
</project>
Now Maven keeps complaining about not being able to download the plugin from our corporate repository (which does not mirror anything on the internet) - it just ignores the repository defined in the POM.