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Enable Auto-Lock and Passcode Lock
If you leave your iPad in a taxi or on an airplane, anyone can pick it up and look through your email, contacts and photos -- and that's probably the least of your worries. A simple way to avoid unnecessary data exposure is with a Passcode Lock. You can enable it in Settings > General > Passcode Lock. A Passcode Lock is useless if a thief grabs your iPad while it's awake.
I also recommend enabling Auto-Lock which locks your iPad after a defined period of inactivity. If you're super security-minded (or General David Patraeus) I recommend that you also disable the 4-digit "Simple Passcode" in lieu of something more complex. According to Agilebits, developer of the excellent 1Password, a four-digit passcode can be cracked in 20 minutes, while an eight character passcode comprised of lowercase letters and the spacebar will take 1,000 years to crack.