got my mk3 in on tuesday, and the word WOW has popped out of my mouth several times as I have used it. after following all these posts as i considered the purchase, its now even fuinnier to read all this stuff. In an hour, off to go shoot a wedding with the mk3, and I am very glad I didn't have to buy several more CF cards like I would have had to with a d800 (sRAW on the mk3 is pretty freaking nice, IQ wise, sRAW looks like it could stand against a full RAW from my 7D.) With the d800, have fun losing the FF capabilities so save some space.
In my local photographers community, the whole d800 thing has come up, and the nikon guys say this ---- for weddings, the d800 would be in the bag 90% of the time, only popping out for the few key moments ----because the file sizes are just way to big for wedding work. In regard, do the math to see which is more worthwhile -- $3000 for a camera that will just be extra weight in your bag 90% of the time, or $3500 on a cam you will use 95% of the time (this only applies to wedding/event shooters - again, if landscapes are your bread and butter the size of d800 would be more manageable).
The d800 really seems to be a niche camera. If I were making my living shooting landscapes, then yeah its a fine choice, but if you work in a variety of settings, you really can't beat the mk3. It's just such a versatile camera.