You can do better than that. User marks iMessage account as 'inactive'. Next time someone tries to send to his address via iMessage, ping their client with an "

You can do better than that. User marks iMessage account as 'inactive'. Next time someone tries to send to his address via iMessage, ping their client with an "invalidate-cache-and-send-as-SMS" status code, and it's done. Everything is handled transparently to the sending user. No matter what, it would require some sort of update.