In the last week of August 2013 the policy of Maven Central has been changed. Files like maven-metadata.xml now require a User-Agent in their request.
Otherwise it will fail with an Access Denied status.
The preferred solution is an adjustment of the policy at Maven Central, but it wouldn't harm if Wagon would set the User-agent.
The attached could be considered as a smoketest.
Update: In the second week of September 2013 the requirement has been reverted.