We have a iMac 20 inch computer that was purchased September 2004.  Beginning the summer of 2008 it began to exhibit some disturbing symptoms.  It would not wake up from sleep mode.  The fans would race at high speed for hours.  Some times the computer would begin to shut itself off.  We took the computer in to the Genius Bar, at an Apple store, September 2008.  The technician looked the computer over and told us they would repair the unit for free even though we did not have any Apple insurance.  It seems that this computer had a serial number that fell within a range of computers with a known problem.  In about a week we got the computer back.  The receipt showed that the logic board and power supply ambient light sensor was replaced for no charge.  We have had the computer running for months now and the repair did solve the problem.   I wonder how many other computer manufacturers would repair a computer 4 years old.