why ext2 does not fragment easily:
http://sdb.suse.de/sdb/en/html/ext2frag.html

why defragmentation may still be useful, and how to avoid that:
http://www-106.ibm.com/developerworks/library/l-partitiontip.html

some more indicators:
http://www.newsforge.com/os/03/10/07/2028234.shtml?tid=23