Look what I found at Canon's CPN website:
"EOS-1D Mark IV – Faster shutter speed needs
As the resolution of cameras increases, and pixel sizes decrease, you may find that there is a need to use faster shutter speeds. The smaller pixels in cameras mean that any slight subject movement during the exposure is more likely to show up as subject blur – this is because it doesn’t have to move as far to affect more pixels. To avoid this just use a faster shutter speed than you would previously have done. For example, if you previously used 1/500sec, you could now try using 1/1000sec or faster."
How many of you knew about this? This may be one reason why some of your images do not look tack sharp at 100% view. The other reasons are severe case of epilepsy or a mis-aligned lens requiring focus micro-adjustment. This is just a Canon disclaimer to avoid thousands of complaints from pixel-peepers like us!