Here is how to get Emoji onto your device. No jailbreak is needed at all. BTW sorry if the pic is small. click on them to enlarge.
All you need to do is go to settings > General > Keyboard > Keyboard. It should then take you to the picture below and if you don't have emoji, simply tap Add New Keyboard
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Then Open game center and tap on your account on the banner on the bottom, view account and on your nickname:
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When the keyboard pops up, tap on the globe key until you get to the emoji, or you can simply hold the globe key until a list pops up and you can then slide your finger to select Emoji.
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There you go you can now add emoji emoticons in your nickname.
But be warned that when you do add emoticons to your nickname, it may be difficult for some to add you to the game center friends and therefore you may need to add them. This is one thing I wouldn't do, even though it is cool to add emoticons to your nickname.
Hope this helps.