Evidently, iPhones are in danger of attack and the only defense currently is to shut them off:

Yesterday, we wrote about a serious security flaw that could allow a remote attacker to take control of the victim’s iPhone by sending a specially constructed SMS message. The vulnerability will be publicly demonstrated and explained in a couple of hours at the Black Hat security conference held in Las Vegas, and yet Apple hasn’t released a patch of any sort.

Be very afraid of the square character, if you receive one in an SMS message, turn your iPhone off.
This really seems like a joke but apparently it’s a danger to your phone.