The answer is that the D800 has the same level of weather proofing as the D700, meaning excellent.
Is it going to die if being submerged? Yes, probably.
Is it going to die if used for extended period of time in rain? Most probably not. When shooting in rain, use a rain cover. If you are concerned about some potential weak points of the body like the memory card door (it does have a seal contrary to the statement above) or battery door, it is very easy to tape these with little to no impact on convenience. I will not mention the flash that is not relevant since it is mostly isolated from the rest of the body.
Our friend Josh seems to have been lucky this time around with his 1Ds3, but even Canon doesn't claim that it should resist being submerged, so I would not consider this as more than one isolated case of luck. I recommend working on balance, walking skills and selection of footwear.
Besides, cheaper/lighter bodies like the d800/5DIII make it realistic to carry a back up on important trips in case shit happens.
Cheers,
Bernard