I just came to know that the urge to keep your legs moving/shaking and the inability to sit with you legs still is actually a disease called Restless Leg Syndrome. This disease effects more than 10% of the Caucasian population. The revenues of the drug Requip (ropinirole hydrochloride) used to treat this disease is about half a billion dollar. (it is a different mater that there is a debate going on whether RLS is actually a serious disorder or has just been hyped up by pharma companies to sell products like Requip (source).)
Severe Restless leg syndrome can drastically reduce the quality of life of the suffers, causing serious interference with sleep and normal day to day activities. Patients are forced to aviod events which requires them to sit idle for long (movies, long drives, long meetings, etc.). Flights are hell for these people.
There is even a help group for people suffering from this disorder.
To find out if you have restless leg syndrome (I pray not) go thru this article
And the next time you find someone shaking legs, please dont get upset.