The RESTEasy manual says "Portable to any app-server/Tomcat that runs on JDK 5 or higher",
but to use the released artifacts on JDK 5, they need to be recompiled from source
because the releases are made with maven-compiler-plugin set to "<target>1.6</target>":
http://resteasy.svn.sourceforge.net/viewvc/resteasy/trunk/jaxrs/pom.xml?revision=1447&view=markup
Line 393

This causes confusion:
http://stackoverflow.com/questions/4608448/can-we-run-resteasy-on-jdk-1-5
http://stackoverflow.com/questions/4553489/jboss-resteasy-for-java-5
and it hurts adoption: we're using CXF in Guvnor right now because Guvnor requires to run on JDK 5 (although we prefer RESTEasy).

Either clarify the manual that they need to be recompiled from source
or change the maven-compiler-plugin to "<target>1.5</target>" and "<target>1.5</target>".