Hey! If you are dealing with HBase + Hadoop 0.20 you should have a look at: http://hadoop.apache.org/hbase/docs/current/api/org/apache/hadoop/hbase/mapred/package-summary.html#package_description ImmutableBytesWritable is a part of HBase and not Hadoop, so maybe you missed something when linking HBase to your MR job.
Regards Erik
