This summer google announced in its third I/O about its ambitious project, interfacing every single hardware in your house to android device.
The Android Open Accessory Development Kit (ADK) is USB based implementation and connects to Arduino (open source hardware) or similar accessories .
The idea is to make android device to communicate with android accessory (mostly Arduino device). The android accessory will act as USB host and power(5V ,500mA) the android device(phone or tablet).This is very simple.Android accessories will be specially designed to attach with Android-powered devices and adhere to a simple protocol (Android accessory protocol) that allows them to detect Android-powered devices that support accessory mode.You can turn any device (light bulb,fan,washing machine, treadmill,toys,coffee machine,TV,garage,BP monitor etc ) to an Android accessory by adding a small embedded (hardware & software) support to it.When your accessory is ready you need to write corresponding app on android device to do meaningful job.
More details can be found at http://developer.android.com/guide/topics/usb/adk.html .
Details on Arduino open hardware/software project can be found at arduino.cc .