iMessage "message not delivered" ... except it is!
Found a solution to the "message not delivered" puzzle that was plaguing my friend and girlfriend. They were sending picture messages and seeing "message not delivered" on their end, even though I usually got the pictures.
It seems that the pictures are being sent to my iPad, not iPhone, in that case. Very weird. But at least there's some logic at work... somewhere! I've found in the past that having iMessage active on both the iPad and iPhone can be problematic so there you go!