Can you bring cell phones on a plane?

Having an airplane is alright, as it won't hurt anything. The problem is having it sending and receiving radio signals; this is mostly harmful if you're making phone calls or sending messages. The reason is because airplanes use radar that is sometimes on the same frequency as cell phones, blackberries, and laptops that have internet connections. With all that background noise, it makes it difficult for the pilots to hear radio communications between planes and with the ground tower. It can also sometimes block radar readings, which can be very dangerous -- especially at night or in bad weather. So when the pilot tells you to turn off your phone, it's in your best interest to do so! =)
Two reasons.

1. There is a chance that the signal may interfere with radio signals the flightcrew is using to communicate with air traffic control.

2. Once the airplane is aloft, your phone probably wont work anyway. Cell-phone towers are optimized for users on the ground, it is rare to get any signal above about 2000 feet above ground level. Even if you did get a signal, the high speed of travel would mean your phone would be switching towers at very short intervals which results in dropped calls and can wreak havoc with the cellular networks.