The average temperature has been falling since it's peak in 1998 and now 2008 temperatures are in and they took a nosedive. Just check out the graph: (Click for bigger image)
The cooling across the global is most likely due to the reduction in solar irradiance which could possibly last years and years to come. Because the solar cycle has been delayed by several years, global cooling will most likely continue for atleast the next two to three years. Our sun may be reaching a new solar minimum which could be equal to the extent of the Dalton Minimum during the Little Ice Age. Also Here see for more about the solar cycle effecting climate.