Interesting article on OS X vs. XP optimization <http://www.xvsxp.com/system-tools/#diskUtils>

Excerpt:

OS X: OS X has no defrag application, however, OS X has implemented four methods of dealing with fragmentation and IO performance.

Automatic file defragmentation works when you open a file in OS X. If the is under 20MB and fragmented, OS X will move it to a contiguous block of space on your hard disk.

Delayed allocation (for HFS+ volumes) allows a number of small allocations to be combined into a single large allocation in one area of the disk.

Hot-File-Adaptive-Clustering tracks frequently used files and moves them to faster portions of your hard disk.

Aggressive read-ahead and write-behind caching reduces the impact associated with minor fragmentation

HFS+ avoids reusing space from deleted files as much as possible, to avoid prematurely filling small areas of recently-freed space, thereby reducing the risk of fragmentation.

Apple's Technical Document regarding optimization: About Disk Optimization. <http://docs.info.apple.com/article.html?artnum=25668>