I don't often post in this bit, but reading some of the comments thought I would throw in my thoughts on this. It has already been 'done to death' on other forums.
It is a corporate 'spin' to use the explanation that economy class syndrome is not confined to economy or even air travel, and that it could happen sitting on a train. It is of course correct that a person could theoretically suffer from DVT whilst watching telly at home, but their are some real differences involved with passengers flying. It is these differences that make DVT more likely in an aircraft, and more likely still in cramped seating.
Factors such as, dehydration, low air pressure, low oxygen tension, drinking alcohol, may all play a part. Most of these are not present whilst watching telly at home.(drinking maybe)
As for mobility, as some people have rightly pointed out, when in the middle of a row of four or five, it can be very difficult to 'just get up and walk about'. Equally, for those who are very tall, the notion of being able to do 'at the seat' exercising is a joke (and the airlines know it), when knees are pinned hard against the seat in front. Why economy? well if you are sitting for some time on a seat (with plenty of room) you naturally tend to move your position regularly, but if you have no room to move, it is less likely you will move, and even then the degree of movement may be restricted.
So, technically, it is incorrect to call DVT economy class syndrome, but from another perspective, all the evidence suggests that it is more likely in cramped conditions on an aircraft. The effect varies from person to person depending on medication (HRT, the pill) and also genetic factors, such as those who have a different version of one of the clotting factors in the blood (factor 5).
If you would like lots of legroom long haul, perhaps the best airline at present is American Airlines, with the largest seat pitch in regular (not premium) economy. Watch out for code shares though. Hope that helps, and I've offended no-one.
If anyone from American is on this site, it would be interesting to know if FA's find it easier to deal with passengers, with less complaints, fewer seat per overhead locker, easier access to rows, etc.. than on other aircraft with less passenger space. I ask that simply because that was the justification used by American for re-configuring their fleet. That is, they said it was one of the commonest complaints from passengers.