Piltdown man,
You are exactly right, as part of the burning process, hydrocarbons and water are a by product. As a result on a cold day the water vapour can condense and result in water dripping from the exhaust. Now in theory a well maintained engine shouldn't produce to much water vapour as all the Oxygen should be burnt off, however if the mixture is not correct then you may get a situation where more water can be produced due to excess oxygen.
Again car exhaust are longer and therefore allow the condensation to occur in the exhaust rather than an airo engine which has a short exhaust and thus blows it into the atmosphere.