From Mac Guides
iMessage is Apple's messaging system. Features include:
- Optional read receipts.
- Real-time notification of when your friend starts composing a reply.
- Automatic detection of other iMessage users, and messages sent to/received from these users will be sent via the iMessage system and not via a txt message. This is useful for anyone that does not have unlimited texting.
- On the iPhone, iMessage must be enabled to send/receive iMessages. If it's not enabled, you can only send/receive text messages.
- You can only send iMessages to other Apple iOS devices, and then only if those devices have iMessage enabled. You cannot send iMessages to non-Apple/iOS devices. However, on the iPhone, if you send an iMessage to another iMessage-enabled device, but iMessage cannot contact the device, iMessage will automatically resend the message as a text message, if the "Send as SMS" option is enabled in Settings->Messages.
- iMessage threads are organized and separated according to each user's "Caller ID". On the iPad and iPod touch, this defaults to an email address. However, on the iPhone, this defaults to the telephone number. For example, if you have both an iPhone and an iPad/iPod touch, this will cause iMessages from your iPhone and iPad/iPod touch to be separated into two different threads, and not one, at the recipient's end. When your recipient replies, this also means that only one device, and not all, tends to get the reply. To make people see only one thread, your iPhone's iMessage Caller ID should be changed to the exact same Caller ID email address used by the iPad/iPod touch. This is set via Settings->Messages->Receive At->Caller ID. Also note that it's not just you that has to make changes; anyone who sends you iMessages should also make these changes, if they have an iPhone. Otherwise, while your sent messages will be seen as one thread by your recipients, their messages sent to you may be seen as two threads at your end.
- If you want received iMessages to go to all of your iOS devices, people must send iMessages to one of your email addresses registered in Settings->Messages->Receive At. If someone sends an iMessage to your iPhone's phone number, the iMessage will go only to your iPhone, and nowhere else. Note that your can associate multiple email addresses with your iMessage account, so that people can send iMessages to any of those email addresses.
- As of January 11, 2012, there still appears to be an issue with lost/stolen (and possibly sold) iPhones, even if the iPhone is wiped. Basically, if iMessage was enabled when the iPhone was lost/stolen/sold, the iPhone can still receive your iMessages (and send iMessages, appearing to be from you). Among other places, this has been reported here on Macrumors and ArsTechnica.
- If you're selling your iPhone, the easiest workaround is supposedly to disable iMessage on the iPhone before wiping it.
- If your iPhone has been lost or stolen, this supposedly works: remote wipe the iPhone, call your carrier to deactivate the SIM card, and then activate a new SIM in a new iPhone (from a Macworld article.