There is a discussion on disk fragmentation in OSX over on Macintouch. Some folks were saying their drives were terribly fragmented, even after installing a new system on a bare drive; after they defragged with Norton Speed Disk, everything ran faster.

Well, I bought the new Norton Utilities, and I am skeptical. On my two-year-old system, I had 0.6% fragmentation. So I defragged anyway, and found a 5%-10% speedup on disk-intensive programs like starting up Classic mode. Big deal, and certainly not worth the time it took.