Great news: According to CPN, "Two new Canon L-series super telephoto lenses – the EF500mm f/4L IS II USM and the EF600mm f/4L IS II USM – are now shipping."
Preorder the Canon EF 500mm f/4.0 L IS II USM Lens at Adorama.
Preorder the Canon EF 600mm f/4.0 L IS II USM Lens at Adorama.