That depends on the type of smoke. If it has a blue tint to it your burning oil, if it's white your burning coolant but the chances are it's just steam from burning off condensation in the exhaust. If it clears after a few miles your OK.
In cold weather, it is probably just steam. Water vapor is a by-product of combustion, but the cold air can't hold the vapor, so it condenses and turns to visible steam as it leaves the exhaust. If the "smoke" persists as the day warms, then you may have a more serious problem.
