Old thread, but the suggestion above works perfectly in JBoss 7.2.0.Final connecting EJB's to JBoss 5.1.0.GA. If you follow the link above:
https://community.jboss.org/wiki/ConnectingToJBoss510GARemoteEJBFromJBoss71
Take that pom file, and deploy it to your Maven repo, reference it from your project.
<dependency>
<groupId>custom.org.jboss</groupId>
<artifactId>jboss-client-jars</artifactId>
<version>5.1.0.GA</version>
<type>pom</type>
</dependency>
Add META-INF\jboss-deployment-structure.xml
<?xml version='1.0' encoding='UTF-8'?>
<jboss-deployment-structure>
<deployment>
<!-- exclusions to allow connecting to JBoss5 clients -->
<exclusions>
<module name="org.picketbox"/>
<module name="org.jboss.logging"/>
</exclusions>
</deployment>
</jboss-deployment-structure>