Please note: If, and only if, you're running Hadoop from straight from the svn *trunk*, as of Sept 12 Hadoop no longer uses the *mapred-default.xml* file.
Please move your mapred-default.xml configuration properties to your *hadoop-site.xml*. Hadoop's configuration also supports the notion of *final* parameters which, once specified, cannot be overridden by subsequently loaded configuration files. These changes will roll-out in the hadoop-0.15.0 release only. Currently, the only documentation on how the new configuration works is at http://issues.apache.org/jira/browse/HADOOP-785. Wiki (or other) documentation, is forthcoming (http://issues.apache.org/jira/browse/HADOOP-1881). thanks, Arun
