Just found this thread in sunny PSP! As an insulin-dependent BA frequent flyer, I find this story both odd and unsettling. I always carry a letter from the Department of Diabetes at my local hospital and will admit to being a bit nervous on one trip when I forgot to bring it.
However, I have never had any kind of problem, with security or anyone else. This includes occasions where the computer has picked me out for a thorough check. US security agents have found my insulin pens, spare needles, lancets, etc and never even questioned what they were or why I had them. Also, while on board, I have been "caught" injecting when a FA appeared at the wrong moment and it has never so much as caused a comment to be made.
These are just my own experiences; I'm not necessarily trying to cast doubt on the story. But from my own experience, I do wonder if there is indeed more to it.
BTW I agree that it would not exactly take a lot of skill these days to put together a fake letter