Since upgrading to Tiger in May, I noticed that the time format in iCal was in the AM/PM format. Being in the emergency services field, and my wife being a nurse, we use the 24-hour format. The 24-hour format was checked in the Date & Time preferences, but there was nothing in the international prefs for this format.
I contacted AppleCare and we deleted files and folders, and restarted the system a few times, but nothing changed. Then someone from the UK stated that his system was displaying 24 hour time in the system and in iCal. So I checked to see if my PowerBook would do the same if I changed the setting to the UK ... and it fixed the problem.
So I copied the time format that is used in the UK settings short (13:00) or medium (13:00:00) time, and I pasted the UK format in my International setting, and that fixed the problem. The only minor annoyance is that now my international settings say Custom and not US.
So as far as I can tell, the US settings don't have a preset for the 24-hour format in the International prefs. Hope this helps someone from pulling out all their hair trying to fix this.

