Al usar ivy:retrieve, no resuelve la dependencia que debe descargarse. La salida se ve así:

Buildfile: C:\Users\Simon\workspace\apollo\build.xml
init:
resolve:

BUILD FAILED
C:\Users\Simon\workspace\apollo\build.xml:42: Problem: failed to create task or type antlib:org.apache.ivy.ant:retrieve
Cause: The name is undefined.
Action: Check the spelling.
Action: Check that any custom tasks/types have been declared.
Action: Check that any <presetdef>/<macrodef> declarations have taken place.
No types or tasks have been defined in this namespace yet

This appears to be an antlib declaration. 
Action: Check that the implementing library exists in one of:
        -C:\Users\Simon\eclipse\plugins\org.apache.ant_1.8.2.v20120109-1030\lib
        -C:\Users\Simon\.ant\lib
        -a directory added on the command line with the -lib argument


Total time: 348 milliseconds

La sección relevante de la build.xml se ve así:

  <target name="resolve" depends="init">
    <ivy:retrieve pattern="${lib}/[artifact]-[revision].[ext]" sync="true" />
  </target>

Aquí también hay una lista de lo que debería estar descargando (desde el build.xml)

  <target name="doc" depends="build">
    <javadoc sourcepath="${src}" classpathref="libraries" access="private" destdir="${doc}" windowtitle="Apollo">
      <doclet name="org.jboss.apiviz.APIviz" pathref="libraries">
        <param name="-sourceclasspath" value="${bin}" />
        <param name="-author" />
        <param name="-version" />
        <param name="-use" />
        <param name="-nopackagediagram" />
      </doclet>
      <doctitle><![CDATA[<h1>Apollo</h1>]]></doctitle>
      <link href="http://download.oracle.com/javase/6/docs/api/" />
      <link href="http://docs.jboss.org/netty/3.2/api/" />
      <link href="http://guava-libraries.googlecode.com/svn/trunk/javadoc/" />
      <link href="http://www.junit.org/apidocs/" />
      <link href="http://commons.apache.org/compress/apidocs/" />
      <link href="http://jruby.org/apidocs/" />
    </javadoc>
  </target>