From Thom Hogan’s Nikon D800 review:
The D800 models use a new 36mp FX sensor. Put another way: 4912 x 7360 pixel images. That makes the D800 (temporarily at least) the highest resolving FX camera available, and by a fairly wide margin. Just so we put that in proper perspective, jumping from 12mp to 36mp does not mean 3x the resolution. Resolution is measured linearly. The actual resolution increase over a D700 is a bit more than 70%. The D800 models also resolve about 20% more than a D3x. Those numbers probably deflated you a bit. They should.
While I’ll never turn down resolution (all else equal), the big visible gains were in the ramp from 3mp to 12mp. When you consider it in print terms, you start to see why. At 360dpi (the native level of the Epson inkjets), a 3mp camera can print a 6″ wide print, a 6mp camera can print an 8″ print, a 12mp camera gets you to an 11″ print, a 24mp camera nets 17″, and the 36mp D800 models get you to 19″. Typically, you can squeak by with 1.5x to 2x those sizes, so those old 3mp cameras were laboring to fill a magazine page, while a 12mp camera did so with ease and could be stretched to a two-page spread without too much trouble. The “sweet spot” for most photographers would be between 12mp and 24mp, depending a bit upon what subjects they shoot. The 36mp of the D800 models is outside most people’s sweet spot.
Since most people don’t print big, once we hit 12mp we got into a good comfort zone for them.
I finally upgraded my 6mp D40 when the ad service said “We need to print that person’s picture in life size. Do you have any files bigger than six megapixels?” Up until then the number of megapixels had never been an issue.