That smoke you can see is fine oil mist. On modern turbofans, the main bearings are sealed by a non contact seal known as a labyrinth seal or knife edge seal. These seals have "fingers" that form a narrow passage with the rotating shaft.
Air is bled from the compressor section and blown across this seal into the bearing cavity, thus, any oil that tries to leave the bearing cavity faces an opposing "wall" of pressurised air.
After oil has passed through and lubricated the bearing, it ends up mixing with this sealing air. This air / oil mixture is then scavenged back to the oil tank. Before it gets to the tank, the mixture is passed through an air / oil separator.
This uses centrifugal forces to de-aerate the air / oil mix, and return liquid oil to the oil tank. The separated air is then ejected overboard from the engine. Obviously, not all the oil can be removed in the centrifugal separator. A small amount of oil is ejected overboard with the sealing air, which can form a visible vapour.
[Edited 2009-08-22 05:43:35]