This method was introduced in Ant 1.6.3. What version of Ant are you using (run "ant -version"). My guess is that you have an older version of Ant somewhere and that is being picked up.
Try this. Edit the ant script, adding "-verbose:class" to the JVM command line. This option causes the JVM to print out each class loaded and the JAR it was loaded from. If you do have an older version of Ant hiding somewhere this should point it out.
Do you by any chance have the CLASSPATH env var set? If so, unset it. Otherwise this will be only the first of several surprises that you will run into. It is always best to set the classpath only on the command line, and hence why most Java apps are started from scripts (who wants to type long classpaths all the time?).