Most people recognize the firm words about turning off your phone. While there still might be a pretty lively debate among field experts about the effects of mobile phones on the electronics inside a plane, this is somthing you don't want to mess around with. It is a legal requirement and this is one of these occasions where you rather want to be safe than sorry.
Oddly enough, most people don't turn off their phone during take-off and landing. Personally I attribute this to unfamiliarity with the device: most people don't realize that simply pressing the power-button doesn't turn off the phone. What does happen is that the phone will enter a sleep-mode, which still can be active for GSM-signals. Even when phones are put into flight mode, they still might be harmfull during take-off and landing. To guarantee a safe take-off and landing, you really need to turn a phone off completely.
Oddly enough, the way to turn off a phone has become a defacto standard. By pressing the power button for a couple seconds (usually around 4 seconds) the phone will ask you to initiate a shutdown procedure. This will guarantee that your phone will not pose a threat to the plane while flying.