The 100-400mm looks too similar to the just released 100-500mm! I somehow doubt this one. Everything else would be great if true.
Take a look at the Sigma 100-400mm F/5-6.3:
Now consider that the Canon version is a F/5.6-7.1 non-L, the rumored Canon version would be even smaller, more in line with a 70-300mm. Those lenses have always been popular, and the people using them really aren't expecting to use it in less than good light already.
The Sigma has a 67mm filter thread, the Canon would easily make it to a 58mm filter thread, which is absolutely tiny and the same as a 18-55mm.
Consider it more of a full-frame version of the EF-S 55-250mm f/5.6 lens, which was equivalent to 400mm in a small, cheap package. I think Canon is developing a lens line up for giving you crop-sensor reach with cheap full frame cameras.