If you are running Hadoop on 64bit CentOS, the reason you saw that warning is the native Hadoop library
$HADOOP_HOME/lib/native/libhadoop.so.1.0.0
was actually compiled on 32 bit. It is just a warning, so it will not impact Hadoop's functionalities.
Here is the way if you do want to eliminate this warning, download the source code of Hadoop and recompile libhadoop.so.1.0.0 on 64bit system, then replace the 32bit one.