Mac OS X 10.6 Server Admin: About Mac OS Extended (Journaled, Case-Sensitive) aka HFSX
HFSX is an extension to HFS Plus and allows volumes to have case-sensitive file and directory names. Case-sensitive names means that you can have two objects whose names differ only by the case of the letters in the same directory at the same time. For example, you could have Bob, BOB, and bob in the same directory as uniquely named files.
A case-sensitive volume is supported as a start volume format. An HFSX file system for Mac OS X Server must be specifically selected when erasing a volume and preparing a disk before initial installation.
If you are planning to use NFS, you should use case-sensitive HFSX.
An HFSX volume can be case sensitive or case insensitive. Case sensitivity (or lack thereof) is global to the volume. The setting applies to all file and directory names on the volume. To determine whether an HFSX volume is case-sensitive, use Disk Utility to examine the format of the disk.