From spec point of view, JSF 2.1 is a maintenance release 2 from JSF 2.0, so any application written for JSF 2.0 will work with 2.1 jars. In resume, with JSF 2.1 the following features were added:

<faces-config xmlns="http://java.sun.com/xml/ns/javaee" 
              xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
              xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
              http://java.sun.com/xml/ns/javaee/web-facesconfig_2_1.xsd"
              version="2.1">
  <faces-config-extension>
    <facelets-processing>
      <file-extension>.jspx</file-extension>
      <process-as>jspx</process-as>
    </facelets-processing>
  </faces-config-extension>
</faces-config>