Canon EOS-1D X
As of the end of last week, Canon is still holding to the end of June for shipments to begin. That's what is still being told to a couple of large retailers.
Canon EF 500 f/4L IS II & EF 600 f/4L IS II
These two lenses are proving to be more elusive to nail down. Current Canon says they're begin shipping in May, however 2 retailers I spoke to have said they're now being told the end of June to the beginning of July for availability.
If any of this changes (and it probably will), I will update.