In what is probably a growing field (example: take a look at nio), ZOMM, just announced their forthcoming Bluetooth leash with alarm for your mobile/cell phone. This personal security device that you keep in your pocket or attach to your keychain that will sound an alarm if you walk away from your phone.
ZOMM also functions as a speakerphone so you don’t even need to get to your phone. It has LED lights for visual notifications, a microphone for two-way communications and a speaker for audible alerts including full speakerphone functionality.
Update: A variety of ZOMM devices are now available starting around $50.