OT -- I'm not sure it will stop you from being mugged, but it will help you not run out of battery life just when you need it most. I bought a Mophie Maxboost Atomic Air Battery Case. It functions as a case and makes the phone appear bigger than an iPhone (so maybe the mugger will pass you by), but it's also an external battery which increases the overall battery capacity of the iPhone. 2200 mAh per the Amazone write-up.

The Mophie Juice Pack Air I have roughly doubles my talk time and it recharges (both iPhone and Mophie) from any mini USB connector, rather than needing a proprietary Apple cable.

I have mine switched to red so it normally uses the iPhone battery and keeps the Mophie as a 100% charged reserve. If I drain the phone for some reason, I can switch to green and the iPhone recharges from the Mophie. Or you can just leave it in green all the time, but as I understand, for some reason the iPhone uses more power when the battery is fully charged, so you actually conserve battery life by letting the iPhone battery draw down -- or so I understand and FWIW... I like a fully charged battery when I walk out the door.