Speculation has started again
We've received a few reports that an announcement for a replacement to the EF 100-400 f/4.5-5.6L IS is “very possible for the end of August”. A few prototypes of the lens are currently in use in Asia.
This has been a lens that has been set for replacement as far back as I can remember. I think it's more probable for release now that Nikon has put their new 80-400 out, which was an area of weakness for them.
We've heard from a few people that Canon is having manufacturing issues and that there's a backlog of new lenses slated for production.