How Do I Send An iMessage From My iPhone?
If you have iOS 5 installed, just open the Messages app. Send a text message just as your normally would and if the individual on the receiving end has iOS 5 installed it will automatically be sent as an iMessage. Sent iMessages appear in blue while a sent SMS appears in green.
If you want to send an iMessage to a friend who is using an iPod touch with iOS 5, you simply send a message to their email address instead of their phone number.