I followed this instructions (
http://www.iphonesavior.com/2009/02/spell-number-app-unleashes-free-iphone-emoji .html) and it worked on my 3G with iOS4:
(1) Be Sure You're on iPhone firmware 3.0, 3.1 or 3.1.2
(2) Download Spell Number from App Store [link]
(3) Launch app and enter this newest code 91929394.59 and close the app (*)
(4) Open Settings > General > Keyboard > International Keyboards >Emoji > See √ Mark
(5) Select the Globe when entering text with any app to add Emoji
(*) before closing the app I tapped on "return"
Hope it will work for you !
Giovanni
PS: I've tried first with 9876.... but didn't work