However, a decent source that's correct about 50% of the time sent me information about upcoming lenses from Canon.
Patents for both optical formulas exist, so it's highly possible. I have a hard time seeing Canon adding to the L lineup until the 8-15 starts shipping.
EF 200-400 f/4L IS 1.4X
We can expect this lens to become an official product with an announcement this fall. Availability in late Q1 2012 I'm told.