In a statement to PDN Online, Nikon USA acknowledged the D4/D800 lockup issue that I reported a month ago (several readers and other websites have reported similar issues). Nikon is currently working on a permanent solution, in the mean time you can turn off both the 'Highlights' and 'RGB Histogram' display options in order to avoid this problem:
"Nikon has received reports of a small number of D4/D800 users who are experiencing 'locking up' issues, where the camera becomes unresponsive until the battery is removed and reinserted. Nikon is investigating a permanent solution to this issue and will update users accordingly. To immediately resolve this issue, users can turn off both the 'Highlights' and 'RGB Histogram' display options in the 'Playback Display Options' sub-menu found in the 'Playback' menu. Nikon apologizes for any inconvenience this may have caused users."
There is another D800 focus/viewfinder issue that was also confirmed by Nikon. Here is an example sent in by a reader: