Maven caches artifact resolution errors by default cached (for 24h IIRC). That might explain why the archetype is not found even if the svn repo was proxied on nexus.
Wiping the : jboss-javaee6-webapp-ear-archetype/7.1.1-Beta1-JBT folder out in the local repo will force maven to resolve the archetype again. You might also want to check Nexus logs to see if the proxy is actually requested for jboss-javaee6-webapp-ear-archetype/7.1.1-Beta1-JBT