A question I am asked daily
We were told ages ago that any replacement of the EF 100-400 f/4.5-5.6L IS would happen after the EF 200-400 f/4L IS 1.4x had begun shipping. That time is nearly upon us.
The new 100-400 does indeed exist and is being actively tested, however Canon is in no rush to announce the lens. We're told that the current 100-400 still sells at an amazing clip and outpaces the 70-300L by a 4 to 1 margin.
Canon may also be waiting to see how the EF 200-400 f/4L IS 1.4x sells, would people really hold off buying it if a new 100-400 was available?
With Nikon announcing their new 80-400, 2013 will probably be the year we finally see the new 100-400 from Canon. Here's a patent for the new 100-400 optical formula, notice that the new design starts at f/4 instead of f/4.5.
Canon EF 100-400 f/4.5-5.6L IS