Scudhunter,
A study amongst pilots probably would not distort the findings. General good health does not reduce the chances of developing a DVT. Any (in)activity which involves immobility for 4 hours or more puts you at risk. I had a DVT some time ago, yet I hold a Class 1 medical and play a great deal of sport. There were a significant number of athletes travelling to/from the Sydney olympics who ended up with DVT's and they, I would suggest would be in the best of physical shape. Also the type of seat is also largely irrelevant. Broadly speaking the same proportion of 1st class passengers as economy passengers end up with DVT - you just get to hear more about the people down the back because there are more of them. A study (published in ther BMJ I think) about 18 months ago showed that approximately 10% of passengers on long haul flights end up with some form of clot in their lower limbs. The vast majority of these are not dangerous and you would not know anything about them - however for the unlucky few..... A possible symptom of a clot is one stiff leg (as opposed to two!) that may develop days later - I have spoken to many long haul travellers over the last few years who have recognised such symptoms, but done nothing about them - they could well have been clots.
The problem is awareness - people need to know that they should be exercising their calf muscles ideally every hour, drink plenty of water, avoid alcohol and caffeine (to prevent dehydration and consequent 'thickening' of the blood making clots more likely) and possibly take aspirin (after seeking a doctors advice) before flight.
And they can affect anybody - fit and unfit alike!