As the posts above if you do allot of short journeys you get condensation in the cylinder head because the engine doesnt get to a high enough temperature to burn this condensation off and it collects as emulsified oil in the cap.
In the worst case it can potentially be a failed head gasket...but you would see emulsified oil in the sump..so check your dip stick, check coolant levels and if all is OK then just take it out for a good old blast. Also just make sure you service the car regularily and change the oil in-between services if neccessary.
Cheers,