The highly-anticipated Canon EF 200-400mm f/4L IS USM lens with the built-in 1.4x extender should be shipping later this week. According to Canon Rumors, the lens has arrived in Canon warehouses in the US and Canada.
Accordingly, we should see the new lens arriving at US retailers later this week. Based on the hype surrounding this lens, including the rave reviews from Olympic photographers, expect there to be some pre-orders stacking up, which could lead to some delays in meeting the demand. You can check availability here at B&H Photo and here at Adorama.