iMessage is a new feature allowing users to send messages over wireless across Apple devices beginning with iOS 5. When I upgrade my iPad to iOS 5, I tried out the new iMessage feature & sent a message to a friend with an iOS 5 device. He received the message and later replied, but his reply only went to my iPad, not my iPhone. I found you can turn on the iMessage feature on your iOS 5 iPhone (to receive messages across multiple devices) by going to Settings, Messages, and turning on iMessage. After you do this, messages (text) will be sent as follows:
If is being sent to another iOS 5 device, it will try to send over wireless or 3G as an “iMessage” message and will show in a blue “bubble” (note: you will not be charged as a text for iMessages)
If it is going to a non iOS 5 device, then it will go out as a “text” message in the traditional green “bubble”.
iMessage by default is linked to your Apple ID, so if you have multiple people using the same Apple ID (for example a family member), then when you turn on iMessage, all of the people using the Apple ID will begin to receive iMessage on all of the devices. You can go into Settings, Messages, Receive At, and add additional emails (beyond the Apple ID) that you want to receive messages from.
Another cool feature of iMessage, if someone is responding to a message, a speech bubble with “…” will appear indicating they are in the middle of typing out a reply…