I presume a bluetooth shield would be necessary in your scenario,
something similar to the one here : http://www.seeedstudio.com/wiki/Bluetooth_Shield
, what would follow next is an android app running bluetooth which there are tons available, especially on google API's page which help in customising your app for your PAN network. This way through your phone you just open up the app, which in turn would override the phone's bluetooth operation and establish comms with the arduino/device.
Although I haven't enough info on your scenario and with my limited experience, I would presume having the device operate off a battery, even so, it would afford utilizing sleep mode, and awakening the device using the app, preserving power and the component's "lifetime".
Hope this helps,