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Indeed, you could just turn it off. Or enable PIN lock on your stuff. In any case, if iMessage was correctly developed, as many other IM platforms, I think it checks for how many retries to send the message and if it can't be sent, it defaults to SMS.
 
In this example, if you are cheating on your wife and leave your iPad at home, then make sure your passcode lock is on and don't share the code with your wife.
 
I have a question, I have my iPhone and an another idevice belonging to another family member under the same Apple ID. If I enable it on my iphone, will that other family member see the conversation?
 
There are quite a few small issues with iMessages so far.

For example I have both my phone and ipad on ios5 with iMessage to see how it handles that. It is quite annoying that everytime you get a message it dings both, and not just once but for every message. Then the lockscreen for the ipad (if I'm actually using the iPhone) has a ton of notifications.

Also, if I leave my iPad at home, it is still connected to iMessage, but if my phone loses data service then the messages only go to the iPad (not defaulting to SMS since I am technically still online).

It is certainly nice but it seems a bit unfinished, I'm hoping they clean it up as we get closer to release.
 
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