At this time, ships use a much dirtier (and cheaper) form of diesel than trucks, for instance, which results in much more sulphur and nitrogen oxides in their emissions than from regular diesel. These new regulations would force them to use cleaner diesel, at least within the buffer zone, and would also ensure they use that cleaner fuel when "idling" in port. This would be an interim measure until a standardized means of implementing "cold ironing" was agreed upon. Then ships would be required to hook up to the power grid when in port and turn off their engines completely.
This from the IMO website -
Marine Environment Protection Committee (MEPC) - 58th session: 6 to 10 October 2008The Marine Environment Protection Committee (MEPC) of the International Maritime Organization (IMO) unanimously adopted amendments to the MARPOL Annex VI regulations to reduce harmful emissions from ships even further, when it met for its 58th session at IMO's London headquarters.
These regulations are due to go into effect January, 2010.