Steve Sailer has suggested that the spread of cell phones in the 1990s helps explain the concurrent drop in crime since youths contemplating a life of crime might think twice if witnesses can easily call the police during the commission of a crime. I found this graph which shows steady growth in phone ownership in America during the 1990s, especially after 1992. Robbery, for example, peaked in 1991 and dropped until around 2000, so there is a rough correspondence here. Plus, my impression is that young people--those most likely to be victims and, presumably, witnesses of crime--were the first to jump on the mobile phone craze.
Roughly half of Americans now have a mobile phone, so one could argue that if their availability were important we should have seen a drop that continued until the present day, instead of the observed plateau. But it might be that beyond, say, a 20-25% cell phone prevalence rate, additional phones would not make much difference. Only 4 random people would need to witness a crime for one to have a phone handy. Such a risk might be too high.