iMessage is an Apple messaging service that allows unlimited text messages via Wi-Fi or 3G/4G between Apple devices with iOS 5. Refer to Determining Message Type - Apple iOS for additional information.
If a user has an MDN ending in four zeros (0000), they are not be able to send iMessages from their phone number. They can still send from an email address. If they wish to send from a phone number immediately, the user needs to change their phone number to a number not ending in 0000.