OnCall Defender Medical Alert (iTunes) is an iPhone app that features 3G connection to a 24 hour security monitoring service. Via a subscription service, you can use your iPhone to send an emergency notification to the service after which local law enforcement or EMT services, depending on the type of alarm, will be dispatched.
The advantage over using 911 is that the monitoring service automatically receives GPS localization of your whereabouts and that you can cancel the emergency call within 15 seconds. The service costs $16.99/month or $9.99 with a one-year subscription. From their website:
The agent will know your name, address, and any specific health conditions or information that you previously entered into your profile. If the police and/or EMS are dispatched, they will already be informed of your situation and your location
A separate product, OnCall Defender Panic Alert performs similar functions but for law enforcement events, a combination app is also available. We recently reported on a study at Florida State University on using Android smartphones to detect falls. While there are dedicated devices using accelerometers to detect falls (Medgadget), in the future, there might be some advantage in integrating detection and communication using a smartphone.