You can input emoji on your iPad without any secret tricks or both non-jailbroken or jailbroken iPad. Here is the instruction:

1. Add Japanese keyboard to your iPad

Go to Settings:

Add Japanese keyboard to your system:

2. Go to any app that you want to input emoji. Switch to Japanese keyboard using the "Globe" icon on the keyboard until it switch to Japanese keyboard.

3. Now in the Japanese keyboard, enter "emoji" without quote, you can see a list of emoji icons are shown on the selection area. You can click the arrows buttons to see more emoji icons.

Yes, this emoji input is not as good as the hidden emoji keyboard on iPhone/iPod Touch, but at least now you can input emoji at no cost in your iPad.

UPDATE: If iOS 4.2.1, you can now enable real emoji keyboard using old spell number easter egg, http://waterworld.com.hk/en/blog/emoji-keyboard-ipad-available-ios-421