Check out this video of a biometrics professor fooling high-end fingerprint scanners with a gelatin finger printed from a digital image of a fingerprint: http://youtu.be/K1Sx_BmfZ8I
Now consider that an attacker with physical access to your phone can either lift your fingerprint from the screen (you have been touching the screen, haven't you?), or can retrieve the software image of your fingerprint by plugging your iphone into their computer.
After reading about how Touch ID actually works, and how it reads the characteristics of the electrical field created between the sensor and the valleys of your finger's living skin tissue, I think it will take something more sophisticated than this professor's gelatin mold. Also, there is no software image of your fingerprint stored on the iPhone, it's a hashed signature of various data points from the electrical field, not a 2D or 3D representation of your fingerprint. Of course I'm not a biometric security expert, just someone who's been reading about Touch ID, so maybe I'm wrong.
Now consider that an attacker with physical access to your phone can either lift your fingerprint from the screen (you have been touching the screen, haven't you?), or can retrieve the software image of your fingerprint by plugging your iphone into their computer.