Update: for iOS 5 and iOS 6 you generally do not need an app to unlock!There are two well known ways: Emoji Free (fastest) and Spell Number (most legitimate). Both are free apps. Spell Number is recommended and described below.
1 Download SPELL NUMBER (Search for This App in the App Store)
2 Enter This Number: 91929394.59
3 Press the home button. Emoji is now unlocked. It's waiting for you to turn it on! Restart your phone if you don't see "Emoji" in the next step.
4 Tap Here: Settings->General->Keyboard->International Keyboards->Japanese
5 Toggle the Switch for Emoji to on!
That's it! you're done. You have it forever.
The Emoji keyboard can be turned off in the keyboard menu. The app just "flips a switch" in the device so once you have Emoji you can delete the app. On any screen that the keyboard is available you will see a little globe near your spacebar. Click it to see emojis, Swipe left and right to see the pages in each category.