I have had this set of cans around two years and it is already failing.  I delicately tried to take the unit apart to repair it (I am very handy with a soldering iron) and I failed to get the unit disassembled.  Fortunately for me, the act of trying to disassemble the unit fixed the problem.  This leads me to believe it was a bad solder joint.  Others have complained about this problem.  For ~$500 I expect the cans to last at least 10 years, like my much cheaper Sennheisers.  Also, the plastic parts of the headband broke within a year although this had no significant effect.
Update: After finally getting them apart, and having to cosmetically damage them to do it, it turns out to be a bad driver.  The solder connections were good.  This is completely unacceptable for a pair of headphones that is only 2 years old.  I will NEVER BUY AKG again!