You use a different part of your finger to touch the iPhone sensor than what you use to touch other things. Hold a glass in one hand, and hold your iPhone in the other with your thumb on your sensor. You'll notice that you are holding the glass with the flat of your thumb, but touching the phone with the tip. The two prints overlap slightly, or not at all.
That means while hackers may be able to lift your thumbprint from you holding other objects, or from other parts of the phone itself, they probably can't get the tip print needed to do bad things on your iPhone.
This means the fingerprint databases held by the NSA, FBI, and border security are largely useless at unlocking your phone: they don't cover the same parts of your fingers.
I point this out in regards to the latest iPhone 5S release with "Touch ID" sensor that reads fingerprints instead of requiring you to type in passwords. We cybersec hackes will be discussing how to break this in the near future, so I thought I'd be the first to make this observation.