It does not appear to be an iphone issue. I switched almost two years ago and most if not all of the people I know have new iphones since then. Some have gone through several but the problem persists. So it appears to me to be an issue with contacts.
It's an issue of how message are routed once you start using iMessage. If the person contacting you has an iPhone and sent you previous messages, the system will continue to try to send that route until they are completely removed from the messaging app. I've even had people tell me they've had people totally delete there contact and and then re-add them. So the combination of deleting them from the messaging app, deleting the contact and re-entering it has usually always resolved the issue. YES, no one should have to jump through those hoops and Apple should resolve this. However, I really don't think this impacts nearly as many folks as some are trying to indicate here. It didn't deserve class action status for several reasons that have already been clarified in this thread.