iMessage has had its fair share of scandal in its brief existence. The biggest current failure mode, as far as I can tell, is if you are using an iPhone then dispose of it for a non-iPhone using the same phone number. What happens is that your friends using iPhones who are trying to send texts to your phone number — as opposed to, say, an email address — will still send them as iMessages even though your phone can no longer receive them.
This, as you can probably imagine, sucks. It only happens if you forget to manually turn off iMessage on your iPhone before you get rid of it. In the past, I'd been good about this — but this time, I forgot.
I've heard a variety of "fixes" since this started happening to me a few weeks ago when I stopped using an iPhone 5. Log into your Apple account and remove the iPhone from it, one common fix says; well, the iPhone wasn't there for me to begin with. Another fix involves doing some magic in Messages in Mountain Lion. Well, I use Lion.
Finally I cried uncle and called Apple. The agent I spoke to, a friendly gentleman, said my situation was "weird" and that he'd need to speak to a senior manager about it. I kind of don't believe that this is an infrequent thing, but can also imagine Apple having the hubris to not bother worrying about the scenario where someone transitions away from the iOS ecosystem. Hard to tell.
After a several minute hold time, he got back to me. The real fix, it turns out, is for each of your contacts to manually delete the entire iMessage conversation thread with you on their phone and then reboot. Afterwards, they can start a new conversation thread, which will test your number for iMessage support and revert to SMS as it should. A true pain in the ass, if I may say so, but at least it's a fix.
He also told me that if I didn't go through this, iMessage will "time out" after 90 days and turn itself off. That's cool, I guess, but a three-month wait to start texting friends again doesn't sound like much fun.