Most US airlines now allow the use of portable electronic devices gate-to-gate. It's awesome - from the second my butt hits the chair to the time I get up at my destination, I can be watching a movie, listening to music, writing a blogpost, or a plethora of other things on iPhone or iPad.
I frequently see people as I travel forget or decline to turn their device's Airplane Mode switch on - do yourself a favor and flip the switch as you take off, or shortly after. This isn't a favor to the FAA or the airline, but for yourself. Your phone will lose a great deal of battery charge trying to search for a cellular signal if the cellular radio is active as you're flying 500 mph through the air.
Turning on Airplane Mode will switch off the cellular radio, Bluetooth, Wi-Fi, and GPS, however once you've enabled Airplane Mode you can turn Wi-Fi and Bluetooth back on for the duration of your flight to allow you to take advantage of in-flight Wi-Fi, Bluetooth keyboards, etc.
Turning on Airplane Mode is easy - just slide up Control Center from the bottom of your screen and tap on the Airplane. Wi-Fi and Bluetooth are right there for easy access as well!
This tip works on all iPhones and iPads and all versions of iOS, however Airplane Mode is only accessible via the Settings prior to iOS 7. Also, this isn't applicable for Wi-Fi only iPads.