"[This is] ...caused by water evaporating in the exhaust stream. Occasional puffs [are] usually from condensation in the exhaust pipes (especially in humid areas) or a water balloon up your stove-pipe. Continual white smoke [is usually due to] a warped head/head gasket and coolant entering your combustion chambers. If you're really unlucky, might be from a cracked head."
Some people have reported that high boost levels may promote white smoke, for some reason. Turning the boost down some cures the problem. This might be related to worn out seals on the turbo, which can leak oil into the exhaust. A bad brake booster can potentially let brake fluid into the vacuum line, which also produces white smoke.