Google has just announced Android Open Accessory which, dares us to get physical. Coming to Honeycomb and Gingerbread devices, developers will be able to tap into the accessory market like never before, making them a part of the Android experience as well. The demo showed how a USB accessory - CardioQuest could be used to monitor heartbeat while exercising.
For now, Android Open Accessory items are only supported through USB but Bluetooth support is coming soon. Ultimately, any accessory built with the ADK will be able to be plugged into your Android device and interact in many different ways, be it through an app or some other UI created specifically for that accessory. We'll have more on this soon but for now, you can check out the ADK documentation provided by Google.