This just started happening today and I'm not the only person experiencing it, 2 of my friends with iPhone 4S' are having the same issues as well, one of them is on 5.1, the other is on 5.0.1, I am running 5.1 The problem is, whenever we send a message out to each other, under the message it'll say "Delivered" for a couple seconds, then it'll switch to "Not Delivered" in red with a (!) icon next to it. Here is the kicker, they are getting my messages, and I am getting theirs. Every single message from both people are saying this, and its happening on their phones as well. More info, they are running on AT&T cellular data in the States. I am forced to run only on wifi as I am deployed overseas. I just tried restarting my phone in hopes it would fix the issue, however now its worse. Now my phone will not recognize my friends as having iMessage, preventing me from messaging them completely. iMessage is really ***** up today.