Enable iPhone Emoji (smiley) icons
I successfully enabled the Emoji keyboard on my iPhone4, making it possible to use a lot of different funny icons in SMS, MMS, email, notes, well, wherever you type something in your phone. There are applications in the app store that will help you do this (cost money), but there is one free application that has a “easter egg” (hidden feature) which will do this for you. This application is called SpellNumber.
Once you download this app, launch it and enter this secret number: 91929394.59 followed by enter (once). Quit the app (pressing the home button) and reboot your phone (iOS 4.x caches available keyboards, so reboot is needed). Once rebooted, enter Settings -> General -> International -> Keyboards -> Add new keyboard. You should have a keyboard named Emoji there. Enable it, and whoops; You have Emoji icons. (you touch the globe icon just beside the spacebar to switch between keyboards)