Jailbreak iPhone users have been able to get Emoji, the Japanese emoticons, on their phones for a while now, but you can now get it even without jailbreaking—albeit a slightly gimped version.
Japanese site iPodTouchLab found out that if you import a specially formatted vcard.vcf address book file with the proper Emojis already inside, you'll be able to type out Emojis a chunk at a time, and delete the ones you don't want. It's a gimpy way of doing it, but it's free. Instructions from WinandMac follow:
Step 1. Download the vcard.vcf here.
Step 2. Import it to your address book program. If you are using Windows, you may want to do it on iPhone directly. Send the vcard.vcf file to yourself. Open in on iPhone's Email.app, click the vcf file and import all the contacts.
Step 3. Go into "Settings" > "General" > "Keyboard" > "Japanese Keyboard". Enable the QWERTY keyboard.
Step 4. In notes or any other program you want. Type "emojia", "emojii" or "emojiu" to select those :) icons.
Step 5. Happy Emoji!
If you want to enable Emoji the "correct" way, where you can pick and choose which emote you're typing one character at a time, you can actually go and download Touch Dial Emoji, an app in the App Store that enables this for $5. Or, you can just jailbreak your phone for free, and enable it yourself via a Cydia app.