The 24-hour trick suggested by Olaf van Es below has been made easier, thanks to the new Swedish Kit. Run the install-script and say no to the Swedish keymap (unless you’re a Swede, maybe 🙂 but accept the changing of the Deskbar and Tracker.
Warning! This works only with the R4.5 and R4.5.1/Intel versions of the Tracker and Deskbar. Attempts to run it on other versions will fail and possibly cause the Deskbar and Tracker to crash.
What follows are the old, manual instructions for those do-it-your-self’ers.

If you want a 24-hour clock in the Deskbar instead of the standard AM/PM display, you can do a bit of surgery on the Deskbar application itself.
Warning: Mucking around with applications in DiskProbe can be lethal to your system — don’t edit anything besides what this tip describes if you don’t know what you’re doing!
In Tracker, open up /boot/beos/system and make a duplicate copy of the Deskbar program. This will be your backup in case anything goes wrong. Next, drag the original Deskbar file onto your DiskProbe application. Hit Alt+F and search on the string: '%I:%M %p' (without the single quotes). Select it and type this string over the top of it: '%H:%M ' (without the single quotes). Whenever you edit strings in DiskProbe, it’s important to keep the number of characters identical — otherwise you could end up shifting the whole program code over to the left or right and breaking things. Thus, pay special attention to the spaces in the replace string above — copy it exactly.
Close DiskProbe, write the changes when it asks you, and the next time you boot you’ll have 24-hour time (alternatively, open a Terminal, then kill Deskbar with your thread manager of choice, then launch it again from Terminal by typing /system/Deskbar &.
After you’ve used the new Deskbar for a while and you’re confident it’s in good working order, you can delete the backup copy you made in the first step.

Bora Ayis (ayis@sasain.com.tr) adds the following:
As Olaf van Es showed how to change the time display from 12-hour to 24-hour, we can also change mm/dd/yy to dd/mm/yy by searching for %m/%d/%y and changing it to %d/%m/%y.