A few of The first batch of cameras had faulty sub mirror assemblies that affected focus accuracy. The sub mirror directs light to the focus sensor.
Later cameras had this fixed as identified by the serial number range (can't remember it but google will tell you) and a blue dot on the box. Canon did a recall in March 2009 for all 1DIII owners anyway to send cameras in for free to be checked and calibrated if required. I sent mine in, it was fine and they also cleaned the sensor at the same time.