Nikon complicated things greatly with flashes in the digital age. None of the film-age flash units work in TTL with digital cameras (there is one small set of exceptions in the D2h/D2hs models). Worse still, there have been two variations in flash in the digital age: D-TTL and i-TTL.
D-TTL came first, and consists primarily of the SB-28DX, SB-50DX, and SB-80DX flash units. Cameras that were solely D-TTL include the D1 series, and the D100. You must use a D-TTL flash with a D-TTL camera to get TTL flash support. The shutter release will lock if you put the wrong flash on the wrong camera set to TTL.
i-TTL was introduced with the D2h (though the D2 series cameras can also use the D-TTL flash units). i-TTL flashes include the SB-300, SB-400, SB-R200, SB-500, SB-600, SB-700, SB-800, SB-900/910, and SB-5000. All DSLR cameras except the D1 series, D2 series, and D100 require i-TTL flashes to perform TTL calculations.
It also really doesn't help that Nikon has pretty much continuously tweaked flash performance in the digital era. The D1 had pretty crummy flash capabilities. Nikon quickly made adjustments to the pre-flash sequence and the monitoring of it in the remainder of the D1 series (D1h, D1x), but this started a continuous process whereby virtually every Nikon DSLR has had a slightly different TTL output tuning in the same controlled situation. Lately the tweaks have been very small, but it's still happening, so my advice has always been to test any new camera and dial in your own flash exposure compensation if you're not happy with the Nikon decisions.
D-TTL Flash Units (again, for D1 series, D100, and D2 series cameras only):
Non-current i-TTL Flash Units (for all other DSLRs):
