Apple's iMessage may be a fine SMS/MMS tool, but as we recently reported, it isn't without its problems.
If you are unlucky enough to find your iPhone stolen or lost, then under some circumstances it is possible that the iPhone can continue receiving iMessages. Even moving your phone number to another handset, changing your Apple ID password, and remote wiping the handset may not stop the problem.
There are two possible fixes, and they are as simple as turning on your iPhone's SIM card PIN and calling your phone carrier...