The device could be used to make a device more secure by reading the owner's heart beat signature, or obvious applications like letting people check their heart rates during a workout.
However, there are also some pretty insidious potential applications.
Besides letting advertisers know metrics about everything from when, where and in what context a viewer watched an ad, which not also tell them whether it set their heart racing?
And just think of the possibilities for a Scientology stress-test app!
On a more humorously speculative note, what if Apple used the device to maintain its identity as a premium brand by only letting fit, healthy people operate their iPhone?
The company can remotely shut off phones, so just be careful if you get one of these newfangled devices not to check in at a Krispy Kreme on Foursquare and then watch your pulse change as your blood pressure goes up and trigger any sort of arbitrary "no fatties" clause in your iPhone service contract.
Jackson West would like to keep his biometrics very much to himself, thank you very much.