[lucrum]

Taken from wikipedia:
"Contrails are formed when hot humid air from the engines mixes with the surrounding air in the atmosphere which is drier and colder (the mixing is a result of turbulence generated by the jet engine exhaust). The rate at which contrails dissipate is entirely dependent upon weather conditions and altitude. If the atmosphere is near saturation, the contrail may exist for some time. Conversely, if the atmosphere is dry, the contrail will dissipate quickly."

First thing that strikes me about that explanation:

Hot humid air from the engines...I might be wrong here, but as far as I know a jet engine does not add humidity to air unless it is capable of providing an internal source of hydrogen and oxygen to make new water molecules that add to the already existing amount in the surrounding air. Combustion in a jet uses oxygen to fuel the flame, therefore if anything the humidity could possibly be lower on the exhaust side of a jet. [corrected below]

From this, I can agree that a normal contrail is formed by turbulence, much like the wing-contrails. It's a condensation of the water vapour existing in the surrounding air because of pressure drops. More humid air surrounding makes thicker and somewhat longer contrails, but if a pressure drop is the cause of condensation it's logical to me that when the pressure once again stabilize the terms for condensation is eliminated and water will go back to being vapour. Hence it will not be long lasting unless the planes makes a stable pressure drop over time, and that would be a good reason to ground all planes pretty quickly.

I don't think we can get very far what regards the "sheep" though, as I find it rather amusing that when, say, a discussion wether a swine-flu shot is good for you or not arise...the sheepling manage to throw facts from WHO, CDC or whatever other governmental official you might find at the time.
Do they not understand that these are the very organizations in question here? :P

Edit: Did some more research and found kerosene to be hydrocarbons, which means the jet engine do produce H2O and CO2 when it is burned. But even so, I still think the explanation given is flawed regarding as to why some trails stay longer than others. Of course the composition of the jet fuel may contain more hydrogen than carbon, I guess it has something to do with the fuel grade.