Should pasta be started in cold or boiling water?
As stated in another answer, Italian tradition is that all pasta is cooked in boiling water. A reasonable explanation for this usage is that it's easier to get the time right this way. 

Pasta is very sensitive to cooking time, and will easily turn from 'al dente' to an overcooked mush if left on the fire a couple of minutes too much. By cooking it in boiling water, you ensure that it cooks in uniform conditions, always at the same temperature, regardless of the starting temperature of the water, the temperature of your kitchen, and the power of your burner. So it is a safer bet that cooking it for the same amount of time will work.

In my experience, the time marked on the package is almost always accurate for pasta cooked in boiling water. So it gives you a useful reference point, which you don't have if you cook it starting from cold water.