When's it coming?
The Canon EF 200-400 f/4L IS 1.4x was mentioned as an in development product by Canon back in February, 2011. A lot of us expected an official product announcement sometime in the following 6 months, however that hasn't happened. The perceived delay is probably due to Canon wanting to get the new 500 & 600 super telephotos into the hands of photographers first. Shipments have started in small numbers, and stock levels should improve in the coming months.
I have been told that an official announcement has been delayed a few times, however we may see it finally announced in June, 2012. There is also a possibility of it not being announced until the pre-Photokina announcements in August or September. You can “almost guarantee” it won't be announced after Photokina I'm told.
Availability? I would expect late 2012 or early 2013. The price I have heard for this lens will be in the are of $10,999 USD.