A recent media highlight in Australia where the news has picked up on various passengers travelling on long haul flights keeling over and dying not long after landing.
The underlying problem is deep venous thrombosis which is clotting of the blood in the deep veins of the legs due to reduced blood flow. These clots can embolize later potentially causing the occasionally life-threathening condition called pulmonary embolus. Moving around tends to cause these clots to break, thus the story of how some people keel over in the airport after the plane lands ...
The major airlines have rushed to update their inflight warnings to include prophylaxis against deep venous thrombosis by suggesting leg exercises, plenty of fluids and walking around the cabin when possible.
The fact of the matter is that some people are more susceptible to deep venous thrombosis than others (Factor V Leiden comes to mind) but the bottom line is that in any situation where you may have restricted movement for more than a few hours at a time, it is a good idea to occasionally get up and walk around, if possible.