Trying to use jboss-as-client for M4, you hit this error because JAF 1.0.2 is not distributable via Maven. Upgrading JAF to 1.1 or 1.1.1 will resolve this problem.

1) javax.activation:activation:jar:1.0.2

Try downloading the file manually from:
http://java.sun.com/products/javabeans/glasgow/jaf.html

Then, install it using the command:
mvn install:install-file -DgroupId=javax.activation -DartifactId=activation -Dversion=1.0.2 -Dpackaging=jar -Dfile=/path/to/file

Alternatively, if you host your own repository you can deploy the file there:
mvn deploy:deploy-file -DgroupId=javax.activation -DartifactId=activation -Dversion=1.0.2 -Dpackaging=jar -Dfile=/path/to/file -Durl=[url] -DrepositoryId=[id]

Path to dependency:
1) org.jboss.weld:weld-core-test:pom:1.1.0-SNAPSHOT
2) org.jboss.test-harness:jboss-test-harness-jboss-as-60:jar:1.1.0-CR6
3) org.jboss.jbossas:jboss-as-client:pom:6.0.0.20100721-M4
4) org.jboss.spec.javax.xml.registry:jboss-jaxr-api_1.0_spec:jar:1.0.0.Beta1
5) javax.activation:activation:jar:1.0.2