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To expand on the comment above, Nikon doesn't support interchangeable viewfinder screens in cameras below the D3S level. Even for the D3S, there aren't any useful Nikon screens available. My theory to explain this state of affairs says that Nikon is afraid photographers would merrily use old AI-S lenses on their cameras if focusing was too comfy. Nikon would rather you bought shiny new autofocus lenses.
The D4 abandons interchangeable screens entirely, ending a tradition that ran unbroken since the model F of 1959. Progress.
Are the factory screens useful for manual focus? Nikon thinks so:
D7000: "With approximately 100% frame coverage in the viewfinder, what you see is what you exactly capture. The specially coated glass pentagonal prism and precision-crafted finder screen offer not only a bright viewfinder image, but also enable you to easily confirm when a subject is in focus."
D300: "The D300S employs Type B BriteView Clear Matte screen Mark II, that ensures easier visual focus recognition in both auto- and manual focus. The viewfinder features an eyepoint of 19.5mm, so photographers can concentrate on capturing the right moment."
D700: "Put your eye to the D700's large circular eyepiece and rediscover why a camera's viewfinder experience cannot be taken for granted. One look through a D700 confirms that superior viewfinder design is at the heart of single-lens reflex camera handling. The Nikon FX format and large pentagonal prism deliver a large, bright viewfinder image, making accurate composition easier for any shooting conditions. What's more, the D700's expertly designed viewfinder enables skilled photographers to confirm focus visually, in either auto or manual focus mode."
D800: "See every important element in your frame clearly and precisely. The D800 offers approx. 100% frame coverage (in FX format) from its slim pentaprism, giving you the visually comfortable FX-format advantage and an unobstructed view when shooting still images. The viewfinder image is not only large and bright — the focusing screen is also carefully designed to help you sense sharp focus intuitively, be it manual or autofocus."
D3S: "The D3S's large prism gives you the FX format visual advantage when you shoot. The viewfinder image is not only large and bright, but the focusing screen is also carefully designed to help you to intuitively sense sharp focus, be it manual or autofocus."
D4: "The D4 offers approx. 100% frame coverage for FX format, with a viewfinder that is designed to minimize visual fatigue over long periods of use. The approx. 0.7x magnification enhances the confirmation of every visual element in the frame. The large, bright viewfinder image and focusing screen are carefully designed to aid precise focusing in both manual and autofocus modes."
My experience with the D300S and D700 wasn't entirely positive. Although their factory screens were fine for focusing slow lenses, I had trouble focusing f/2 lenses with high reliability, especially on the D300S. To improve matters, I installed KatzEye Optics split-prism screens in both cameras. These improve accuracy and ease of focus compared to the factory screens, but since they transmit light differently in the central split-prism area, they can affect spot-metering accuracy. (I didn't notice any changes, but I didn't check very carefully.)
The installation process involves tweezers, dust phobia, and leveraging open a tight metal clip millimetres from a delicate focusing screen. But I presume that's the case for cameras with official interchangeable screens, too. I managed, and I'm no great mechanic.