I found out that if you set the Menu bar clock to display 24-hour in in OS X 10.5, it only affects that display. In menus and most everywhere else if you desire the time/date format to include 24-hour clock formatting, you must set up each parameter in 'International' in System Preferences (hours format, AM/PM). Once set there, the format applies universally (regardless how your Menu Bar clock is set). Here's how...
Setting for 24-hour clock in Leopard requires setting up the clock hours format (drop down pattern choices) to 00-23 hours (0-23 hours) AND deleting both 'AM' & 'PM' in the appropriate boxes.
Is there any more direct way to change the system clock from AM/PM 12-hour clock to 'military time' (aka 24-hour clock), in Leopard?
Curiously, in Snow Leopard (SL) all the above is changed by 'tweaks' in Clock settings that carry that selection to the international settings when the box for '24 hour clock' is checked. Also in SL, the (System Preferences>)Language & Text settings for Medium, Long, Full have become irrelevant (and don't affect the displayed info in menus or menu bar clock) when 24-hour clock is enabled in Time & Date. There now, if you select 24 hour clock, the AM/PM display becomes grayed out. Just some more interesting little 'tweaks'.
If you use use 24-hour clock (sometime soon) and need to set it with some machines on SL, and some on Leopard, the differences may get confusing.