On the Nokia N8 you can launch a Qt application and call the following:
QString timeFormat = QLocale::system().timeFormat(QLocale::LongFormat);
The time format string on a phone with the settings set to 24h clock will be:
h:m:s
Then without exiting the application, multitask out and change the settings in the phone to use a 12h clock with am and pm. Call the same code again:
QString timeFormat = QLocale::system().timeFormat(QLocale::LongFormat);
The time format string is still "h:m:s". Restart the app and look at the time format string again, it is now:
h:m:s AP
As it should be.

It seems as the system locale in Qt is not refreshed when things change in the phone.