David C - Stuart actually raises a very important point. Passengers should not be boarding aircraft if they are medically unfit to do so. He is clearly not talking about unpredictable medical emergencies, but those that should have been reasonably forseeable.
As an example: On a particular flight, a passenger collapsed and had difficulty breathing. The crew gave first aid, and a doctor was sought, and on the advise of that doctor (who was able to get a bit of medical history once the passenger started to improve) the flight was able to continue to its destination. If that doctor had not been on board, the crew would have had no choice but to divert the aircraft based on the cabin crew reports. So much so that the diversion routing had already been loaded.
It was subsequently discovered that this passenger had a medical condition which required supplemental oxygen on the ground! Thats right, on the ground (let alone at 7000ft), the passenger required oxygen. However, the passenger (and their family) failed to notify the airline, get medical clearance, or to arrange for oxygen to be available on the flight.
Now, and this I believe is what Stuart is getting at - why should the passenger not be held accountable for what is nothing short of irresponsible behaviour? Had the aircraft diverted it would have been for completely avoidable reasons, and would have cost a small fortune.
This doesn't for a second mean that, at the time, you are concerned for anything other than the passengers well-being. And 99% of the passengers are similarly ok with the diversion because of the needs of the passenger. But what would those passengers think if they knew that it was entirely avoidable except for the actions of the passenger and/or their family?
Stuart - you ARE required to complete such a form in many circumstances.
Qantas' one is
http://www.qantas.com.au/infodetail/...l/mediform.pdf
All that said, the cost and low likelihood of success would make chasing a passenger for the costs of a diversion impractical.