The Nikon D4 camera is again open for pre-order on Amazon with a release date of March 20th, 2012. Amazon stopped taking pre-orders just few hours after the D4 announcement. They probably now know how many cameras they will get in the first shipment and started taking pre-orders again.
Update: Nikon D800 and D800E pre-orders are also opened:
The Nikon D800 is also listed with a March 20th release date. No release date is given for the D800E: