OK. Even I'll admit that this looks really petty, but let's face it - I've got a point.
Just look at this from a simple usability point of view. Nobody uses the 24 hour clock in Real Life. If I'm arranging to meet someone, it's never at "18 hundred hours" or "1815", but "6 o'clock" or "quarter past". I can tell if it's in the afternoon or not by checking to see if the sun is up (if it's 2 o'clock and dark, it's am. If it's light, pm. Easy.) I have to translate to 12 hour every time I look at it. I know it's not hard, but it's like reading in a foreign language - it gets in the way of what I'm trying to do; it makes me think about the format of the time, rather than what I'm doing, like checking to see if I'm late for an appointment.
Timetables are the only place I'm willing to concede that it's useful (and the military, well, they'll do what they want to anyway...). And then they're not doing anything that you couldn't do with "am" and "pm". In fact, I've started seeing timetables do just that, and they're much easier to read. And I don't understand why cinema's need to use the 24 hour clock either - they're not open at 8am, so you could tell just from the context what time the film starts. And using the 24 hour clock is just too formal - I only want to see a movie.
Now, I'm aware that this is just a cultural thing. A Frenchman I know used to jot down meeting times in 24 hour notation. It was natural to him - he used it. Here in the UK, we don't use it.
So, what does actually wind me up about all of this is that every video, radio, cooker, alarm clock or any other electrical item you can mention will have a 24 hour clock. These items are supposed to be labour saving, user friendly devices, and yet they don't even present the time to us in a format that we want. I'd really like to hear why a video designer chose to implement a 24 hour clock...
Even Windows gets in on the act - it's default time formatting for the UK is 24 hour. So even everyone's computer is adding to the trouble.
Petty? Perhaps. Poor usability? Oh yes. Bad design? Definitely. Know your target audience. Build appropriately.
(Or the alternative moral: if you're designing a new DVD player for the UK, please make it display the clock in 12 hour. You'll have at least one customer.)