Danny:
I'm no medical expert but I think a study of pilots would be dismissed for a couple of reasons.
Pilots are required to pass strict medicals on a regular basis, and I suspect this general level of good health would distort any results.
Secondly your average pilot isn't sat in your typical airline seat -- be that first-class or economy -- and I reckon doesn't replicate the same typical movements of a passenger.
These might be significant, or they might not. Either way, this whole DVT thing seems to be difficult enough to pin down without introducing the difference in pilot/passenger situations as a possible influence.
Incidentally, there was an interesting news article published a few days ago citing a study which shows that DVT mortality rates from air travel are no higher than those for the general population...
...and it didn't even mention the phrase 'economy class syndrome'
