I just switched to Maven 3.1 and saw that Findbugs was not reported on the page of the job anymore. When I switch back to Maven 3.0 it works like always.

With Maven 3.1 I found two different warnings in the Logs:
WARN org.apache.maven.eventspy.internal.EventSpyDispatcher - Failed to notify spy hudson.maven.Maven3Builder$JenkinsEventSpy: Failed to load edu.umd.cs.findbugs.detect.TestASM

WARN org.apache.maven.eventspy.internal.EventSpyDispatcher - Failed to notify spy hudson.maven.Maven3Builder$JenkinsEventSpy: Could not initialize class edu.umd.cs.findbugs.SortedBugCollection

Jenkins version: 1.526
Jenkins Findbugs Plugin version: 4.49
Findbugs Maven Plugin version: 2.5.2