Yep, only noticed this yesterday, tried a whole bunch of things, such as changing setting on Date & Time on the computer, changing International settings, changing the iPhone's "Set Automatically" both to ON and OFF to no avail.
If you try other things, let us know, I just filed a bug report and I can include more info, there's probably others that have done the same by now and I would think some of the developers themselves might have also caught this, it's a pretty evident one.
It's a question of personal preference, but since I've used 24-Hour format for so long on everything I can, I can't stand the 12-hour format anymore!
Hoping for the best
Edit: Updated my Mac Model, don't know why it had the old one...
iMac Intel 2Ghz 17in OS 10.4.10 / iPhone 8GB v.1.1.1