Consumer primes, especially on the telephoto side seem to be on their way out. I have let a few friends use my 300mm f4 IS when we are out shooting together, and the first question they ask is "Where is the zoom?". I think the current 70-300 is the best we'll see for a while. (Don't hate me, this is not what I voted for, just what i think is happening)
That said, to those dreaming of a 400mm f4 IS, Don't forget, Canon already makes one. The DO. (A green ring for goodness sake, bleh!) Not my favorite either, but do you really think Canon is going to come up with a $2000 400mm f4 IS, to sell next to their $6000 400mm f4 IS DO?