According to a New York Times Sunday Book Review Essay by Erica Wagner [thanks Elizabeth!], the cell phone is the bane of the modern novel's existence and the reason that historical fiction is so popular.
How does one write a great mystery novel in the age of the cell phone? These days you can call the police from anywhere, track down a missing person by calling their cell, or speed up your sleuthing by searching for clues while making phone calls at the same time. It's tragic!
And it goes beyond just the novel. Our lives are too traceable and well-documented anymore (take Twitter for example - ack!) So let's add some mystery! Leave your cell phone at home while walking through dark alleys, let the battery run dead before you find yourself with an urgent need to call 9-1-1, and don't answer your phone when you know people are wondering where on earth you are. :)