Here is a smartphone armband that lets you exercise with your device and keep an eye on your heart rate. Shapeheart is adjustable and has a magnetic phone case. It has an optical sensor with Bluetooth and 25 hours of battery. You can use it with your favorite running app to check your pace, distance, and heart rate.
Shapeheart lets you attach and remove your phone easily. It also has a touchscreen compartment. Expected to ship in November.