A new iPhone application, currently awaiting approval from Apple, will monitor your heart rate via the built-in microphone (through either the onboard mic on the iPhone or through the mic/headset combo). The developer has released the above video demonstration.
In addition to monitoring your heart rate using the microphone, you can also count your bpm (beats per minute) using the voice prompts. The application then stores your heart rate for later reference. We can't wait to see this application hit the app store!