I just read about this in a new book I recently downloaded to my iPad titled '13 Things That Don't Make Sense: The Most Baffling Scientific Mysteries of Our Time', by Michael Brooks.
For more information about the 'Pioneer Anomaly' (including what might finally be an explanation, which has only been proposed since the book was written in 2008 ), go to:
http://en.wikipedia.org/wiki/Pioneer_anomaly

OCU