What is interesting to me always is that my experience trying to send iMessages to people is usually the opposite: if someone's phone runs out of batteries or they start a phone call (on CDMA, which can't do voice and data simultaneously) I nearly instantly am forced to send them text messages. I also have found the "Delivered" notices very reliable: AFAIK they require a device to actually receive the message. Note however that it is "a device": if you have iMessage associated with another random device, it might be receiving your messages for you; I would be very interested in knowing if these people cataloged all devices they have from Apple (including Macs) and logged out of all of them. (It could, of course, just be a bug; but it at least doesn't seem to be some fundamental aspect of the design that it permanently hijacks messages.)