For those who still think Bluetooth on iOS is hard to do, you haven’t been paying attention to Bluetooth Low Energy. Apple’s Developer portal provides specs and sample code to build accessories that use Bluetooth LE.
I have been playing with the LightBlue app that can be freely purchased from the iOS and Mac App Stores. Their blog describes how the tool can be used to test devices.
Google’s Android is moving in the same direction.
Texas Instruments provides the SensorTag kit and software for peripheral development, and it works with Android and iOS.