iMessage was just announced. Its a new feature that Apple has introduced in iOS 5, apart from the amazing new iOS 5 Notification Center. If you're familiar with BlackBerry devices and how BBM works, you won't be surprised by iMessage. iMessage will work via 3G or WiFi and will enable you to send messages, pictures and videos to any iPhone, iPad or iPod touch running iOS 5.
Let me tell you how this is similar to the BlackBerry Messenger. Whatever iMessage does, BBM has been doing it for decades, with the exception of working on WiFi. iMessage gives you read/delivery receipts, real time typing indication and the ability to send photos and videos. Also, if you've started the conversation on your iPad, you can pick it up from the point where you left it on your any other iOS device.
The limitation of BBM is that it needs a BIS data plan, whereas iMessage will work via both 3G and WiFi. Meaning iMessage is not just limited to the iPhone, it can be used on the iPad and iPod touch as well.
This is also going to make it easier for those who never exactly switched over from BlackBerry to iPhone just because they would lose their BBM contacts. Now even if their BBM contacts have an iPad or iPod touch, they can easily stay in touch with them, just as they would via BBM, using iMessage, provided that their iOS devices are running iOS 5.