Per this ArsMac thread, Panther features “Hot-File-Adaptive-Clustering”.

In plainer terms, the OS defragments HFS+ filesystems automatically, if the files being used are smaller than 20MB.

Be sure to read the thread, for it contains nifty snippets of the actual kernel code, since the core OS, Darwin, is Open Source.

And yeah, if you’ve been living in a cave or something and haven’t already had some Mac fanatic splatter drool on you about it already, Expos?? really does rock.