Imessages are actually contained in the messages app, when you send a message to someone, it will actually check to see if the person you are sending the message to has an iOS 5 device, if they do it will send the message in a blue bubble
If using an iPod or iPad, you can use imessages by using your apple ID. There is a log in box in messages under in settings where you can log in and I think we can add any id you want. Not sure on this though!
Sent from my iPad using iPF