What many of us are waiting for ...
Canon Rumours is reporting that the replacement 100-400 is being tested 'in the wild' with a possible announcement toward the end of the year. Most obvious difference is that twist ring replaces push pull, but I think we can assume the latest IS system will be incorporated. Those 2 features alone would make it of interest, but one would hope/assume improved optics as well.
Sadly it appears to still be a variable aperture f4-5.6, but to be honest, with improving ISO performance, slow lenses don't bother me too much.
Price estimated at us$2.5K + ... a lot of money, but at least more realistic than the 12K for the 200-400