In my experience there are situations where an OS X drive can become severely fragmented.
The Macworld article referenced previously states that
routine defragmentation is not required. It goes on to say:
File fragmentation is an issue primarily for people with almost-full hard drives and people who work with very large files or large amounts of data (for example, video and some image files). If your drive is almost full, a better option is to free up space or upgrade to a larger drive. People who truly need to defragment—or optimize, which is defragmentation that also attempts to position particular types of files for optimal performance—can use utilities such as Prosoft Engineering’s $99 Drive Genius 2 or Micromat’s upcoming $98 TechTool Pro 5. (You may also hear about directory fragmentation. This is different from file fragmentation, and is addressed by utilities such as DiskWarrior.)
You can download and run iDefrag to see if your drive is indeed fragmented. If so, you can pay for a license and fix the drive. iDefrag costs $30.95. It's the least expensive software that I know of.
You can also create a bootable clone of the drive, reformat it and copy the data from the clone. The result will be an unfragmented drive. But it won't be optimized like it would be if you used iDefrag and similar utilities.
iDefrag
http://www.coriolis-systems.com/iDefrag.php
Macworld UK Review
http://www.macworld.co.uk/digitallifestyle/reviews/?reviewid=3228570