The rumored-for-about-a-decade-replacement for a 16-year-old, very popular lens is certain to be in very high demand. While I do not yet have insight into the initial inventory levels for this lens, I highly recommend that you preorder immediately if you want one of these highly anticipated lenses anytime soon. I expect long lines to form for this one.
B&H is accepting Canon EF 100-400mm f/4.5-5.6L IS II USM Lens preorders. Adorama and Amazon are also accepting preorders.