I need to prompt the user to enter his/her zip code at certain times in my iPhone application. I cannot store it or get it from the user's current location.

What is the optimal input method?

I started with a 5 wheel picker. This seemed like a bad direction, so I opted for a PIN-like entry screen. My implepentation is about the same as the 'enter passcode' screen you see when unlocking the iphone - including automatically ending input on the 5th digit instead of providing a return/enter button.

Does this seem optimal? Or is there a better way?

Note: I only need to accept a 5 digit US zip code.