After contacting the lead architect for the FlexPMD project, he recommended that I build from flex-pmd-java-parent instead of flex-pmd-parent. Having made that change the build succeeded with Maven 2.
I'll point out that the Developer Documentation does mention the version of Maven required, and it suggests building form flex-pmd-parent rather than flex-pmd-java-parent.
Here's the tail of my build results showing success:
[INFO] [jar:jar {execution: default-jar}]
[INFO] Building jar: /opt/flexpmd/trunk/flex-pmd-metrics-maven-plugin/target/flex-pmd-metrics-maven-plugin-1.3 -SNAPSHOT.jar
[INFO] [plugin:addPluginArtifactMetadata {execution: default-addPluginArtifactMetadata}]
[INFO] [license:check {execution: default}]
[INFO] Checking licenses...
[INFO]
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO] ------------------------------------------------------------------------
[INFO] Adobe Flex PMD Java Parent ............................ SUCCESS [2.930s]
[INFO] As3 Parser API ........................................ SUCCESS [2.847s]
[INFO] As3 Maven plugin Utils ................................ SUCCESS [0.791s]
[INFO] Adobe Flex PMD RuleSet test resources ................. SUCCESS [1.482s]
[INFO] Adobe Flex PMD Files .................................. SUCCESS [3.772s]
[INFO] As3 Parser ............................................ SUCCESS [1.993s]
[INFO] Adobe Flex PMD RuleSet API ............................ SUCCESS [4.163s]
[INFO] Adobe Flex PMD RuleSet ................................ SUCCESS [23.814s]
[INFO] Adobe Flex CPD (Copy and Past Detector) ............... SUCCESS [2.668s]
[INFO] Adobe Flex CPD Ant task ............................... SUCCESS [9.320s]
[INFO] Adobe Flex PMD Core ................................... SUCCESS [9.003s]
[INFO] Adobe Flex CPD Maven plugin ........................... SUCCESS [3.718s]
[INFO] Adobe Flex PMD Command line API ....................... SUCCESS [2.308s]
[INFO] Adobe Flex CPD Command line ........................... SUCCESS [4.136s]
[INFO] Adobe Flex PMD Ant task ............................... SUCCESS [11.027s]
[INFO] Adobe Flex PMD Maven plugin ........................... SUCCESS [12.468s]
[INFO] Adobe Flex PMD Command line ........................... SUCCESS [5.865s]
[INFO] Adobe Flex Metrics .................................... SUCCESS [5.169s]
[INFO] Adobe Flex Metrics command line ....................... SUCCESS [3.986s]
[INFO] Adobe Flex Metrics Ant task ........................... SUCCESS [4.423s]
[INFO] Adobe Flex Metrics Maven plugin ....................... SUCCESS [7.796s]
[INFO] ------------------------------------------------------------------------
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESSFUL
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 2 minutes 5 seconds
[INFO] Finished at: Thu Aug 02 10:07:27 EDT 2012
[INFO] Final Memory: 49M/101M
[INFO] ------------------------------------------------------------------------