Journaling allow the system to recover more easily from crashes and protect against corruption by having a "journal" (a buffer where changes are first made and then afterwards written to disk) and makes recoveries much faster when using tools like fsck. In exchange for this, extreme disk usage could have an impact (up to 10%) on performance. So, it's not recommended if your machine moves a lot of files around unless it's a file server, in which case it is *strongly* recommended (the priority in servers is safety of the files, not the speed of the disk).
To enable journaling on OS X Client, use the Terminal command:
% sudo diskutil enableJournal /There does not appear to be a man page, but 'diskutil --help' will get a rudimentary list of commands.
WARNING: This could very well mangle the filesytstem. Do not try it if you're not comfortable using root, the terminal or if you don't have everything you care for backed up safely somewhere (I use my iPod for this).
[Editor's note: The release of OS X 10.2.2 earlier today helped set a new record of sorts on macosxhints! The above hint regarding the journaling file system was submitted by 14 different users within a span of 45 minutes! Since 'eduo' was first in the queue, it's his hint that I chose to run, but thanks to everyone for being on the ball on this one! Edited: I fixed the command and removed the reference to the 'man' pages, as there don't seem to be any...]

