A new patent application has turned up from Apple that sounds interesting. The application is for sensors that would potentially let the user sign in just by picking the iPhone up.
The system uses a series of sensors in the case of the phone that would read the person's heart rate while holding the phone. The iPhone would be able to tell if the user was authorized by analyzing the heartbeat.
Exactly how the analysis would work is unclear. I wonder if you took set up your login while resting and then tried to access the phone with your heart rate increased from exercise if you would let you in.