A few people are reporting this problem due to an old cache being loaded. From the installation instructions, here is the fix:
If you are upgrading from a previous version of Maven, you should remove your local plugin directory, specified by MAVEN_HOME_LOCAL/plugins. The default location for this is HOME/.maven/plugins
The next Maven release will absolutely recognise this and avoid the need to delete ~/.maven/plugins. I’ve also fixed the corruption of the cache that can occur by running two instances simultaneously.
Brett Porter 