A new 100-400L was what I was waiting for, but no longer. I suspect that if it is ever actually produced it will cost at least more than twice the $1069 price tag of the Tamron, and it will still stop at 400mm.
I think it will stop at 560mm, with a 1.4x TC attached, and likely be better at 560mm f/8 than the Tamron is at 600mm f/8. From what I've seen of tests, the Tamron really needs to be stopped down to f/8 at 600mm or there isn't much advantage over what you get at 400mm from the same lens.
So, it'll be 100-400/4.5-5.6 and 140-560/5.6-8 versus 150-600/5-8. And I'll bet the Canon will focus faster, be smaller and lighter, and have better handling and IS. And at twice the cost, of course.
But, we'll see if it happens or not soon enough.