Apple has introduced Emoji keyboard in iOS 5, which's now a standard international keyboard accessible in any app. "Originating from Japan, the emoji picture characters are standardized and many phones support them without requiring a Japanese operator (heck, even Gmail supports emojis)."
Here's how to enable Emoji international keyboard:
"Just go to the Settings app, navigate to General > Keyboard > International Keyboards and tap Add New Keyboard. From the list of international keyboard layouts choose Emoji. From now on, whenever you want to add some flair to your messages, switch to the beautiful Emoji layout by holding the globe key on your virtual keyboard. You can swipe through different emoticons, choose between mood categories on the bottom and access the section with your most recently and frequently used emoticons. If you instant message a lot, and especially if you're excited about Apple's new iMessage service, the Emoji keyboard is Godsent."