I'm glad this thread has cropped up. We were nearly caught on the hop with Advanced Passenger Information, booked to go to Fuerteventura on 3rd March and only through talking to one of my friends did I find out I had to enter my API details. I wasn't advised by Thomas Cook at all that I had to do this and would probably not be allowed on the flight if i hadn't inputted it on their website.
I don't think API has been well publicised or advertised at all and there will be people who get caught out on this.