UPDATE: Emoji is available in Cydia.
UPDATE: Method to use Emoji without jailbreak: http://designer.dlinkddns.com/blog/?p=107
http://steventroughtonsmith.blogspot.com/2008/11/how-to-enable-emoji-systemwide.html
Here is the already modified plist: http://localhostr.com/files/cd6a49/com.apple.Preferences.plist.zip
UPDATE: Method to use Emoji without jailbreak: http://designer.dlinkddns.com/blog/?p=107
You need to edit the file /User/Library/Preferences/com.apple.Preferences.plist on the device -> whether you use a jailbreak to achieve this or merely some iTunes backup editor is up to you.
Add the following boolean key as 'true':
KeyboardEmojiEverywhere
Then merely go to the Keyboards section of the Settings app, hit Japanese, and turn on Emoji. Will work for any text field/view in the OS, including on websites, AND including the titles of items on SpringBoard (e.g. if you save a bookmark to the home screen).
Have fun!
http://steventroughtonsmith.blogspot.com/2008/11/how-to-enable-emoji-systemwide.html
Here is the already modified plist: http://localhostr.com/files/cd6a49/com.apple.Preferences.plist.zip