Jnlp Specfication allows you to nest different resources depending on which jre is used as follows:
   <resources>

      <j2se version="1.4">
         <resources>
            <jar href="draw.jar" main="true" download="eager"/>
         </resources>
      </j2se>
      <j2se version="1.5">
         <resources>
            <jar href="draw15.jar" main="true" download="eager"/>
         </resources>
      </j2se>
      <j2se version="1.6">
         <resources>
            <jar href="draw16.jar" main="true" download="eager"/>
         </resources>
      </j2se>
   </resources>

however in both 1.5.0 and 6.0 the main jar cannot be found if it is in such a nested resource. (using javaws 1.4.2 with such a jnlp file causes a core dump on solaris) Resources other than the main jar (such as properties and non-main jars) seem to work.