In contrary to the specification in AS 4.2 a Bean Developer must provide a String value for an env-entry-value.

So the following deployment descriptor will not work:
<?xml version="1.0" encoding="UTF-8"?>
<ejb-jar xmlns="http://java.sun.com/xml/ns/javaee"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  version="3.0"
  xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/ejb-jar_3_0.xsd">
  <enterprise-beans>
    <session>
      <ejb-name>OptionalEnvEntryBean</ejb-name>
      <env-entry>
        <description>default env-entry</description>
        <env-entry-name>entry</env-entry-name>
        <env-entry-type>java.lang.Double</env-entry-type>
        <injection-target>
          <injection-target-class>org.jboss.ejb3.test.ejbthree985.OptionalEnvEntryBean</injection-target-class>
          <injection-target-name>entry</injection-target-name>
        </injection-target>
      </env-entry>
    </session>
  </enterprise-beans>
</ejb-jar>