The app, iWitness, is free to download for iPhone users (it's coming to Android soon), but requires a subscription to use. Basically, it works like this:
When you're about to enter a "potentially threatening environment" you launch the app and "arm" it. A blinking light emanates from the phone, alerting you -- and hopefully anybody up to no good -- that the phone is armed. If you feel threatened, you then touch your screen and your phone starts capturing audio and video and sends it to a secure server that law enforcement can access. Your GPS coordinates are also recorded and embedded in the video.
If the situation escalates, you can touch the screen again and 911 is automatically called, a text is sent to as many as six preselected contacts, and your phone puts out a "loud" alarm (by loud the developer means as loud as your little iPhone speakers can get).
On the surface, the app sounds like it might have some appeal. But as I said, one of the catches is that you have to pay $3.99 a month to use the service (you sign up at the iWitness Web site).
That may be fine for people who often find themselves in potentially dangerous environments, and iWitness is marketing the service to "women, students, night shift workers, the elderly, and many others who may feel unsafe at times." But the price may be steep for folks who do their best to avoid threatening environments and will rarely, if ever, use it.
The other issue is that cell service still isn't ubiquitous, so you may not be able to transmit your audio/video should you get into a dicey situation. Also, you may not have time to explain what iWitness does to a perpetrator -- or actually arm the app in time -- before you become a victim of a crime (yes, a lot of crime takes place in what would seem like perfectly safe environments).
iWitness counters that argument by saying that law enforcement officials have said that in "most cases a person will hear, see, or be engaged by an assailant before they strike."
Anyway, I'll let you guys weigh in on on how effective you think this might be at deterring a crime. Ideally, of course, I'd test it out and try to get some feedback from criminals out in the field, but I'll leave that to intrepid CNET editor Jason Parker, who's in charge of app coverage at CNET.