Apple - Display seconds in addition to hours and minutes on the Clock
If you look at the icon for the Clock app on your home screen it has a second hand. For most cases this is sufficient for me.
Is there anything I can do to cause the clock to display seconds in addition to minutes and hours?
In the default, non-jailbroken OS? I don't think so.
In the status bar on a jailbroken iPhone? Yes. It's called StatusBarSeconds.