IF Nikon were to do a FX in a D90 body, it would have to be even more crippled than a D400. . .so, I would guess they'd remove the AF motor and FORCE use of their AF-S lenses. . .which is kind of smart, since most pro's have AF-D lenses and would be forced to stick with the D700/800/D3s lineage. . .
They could EVEN still keep a D90 body around with a DX sensor and a AF motor. . .so there would be a highly segmented market:
D3000-5000: 12mp DX no AF motor but D-movie (not on 3k); $550
D90 series: 12mp DX WITH AF motor; $800
D9000 series: 12mp FX with NO AF motor; $1400
D400: 16 or 18mp DX with AF motor (100% VF); $1700
D700s/800: 12mp FX WITH AF motor; $2600
D3s/D4: 16 or 18mp FX with AF motor (100% VF); $5200
I think I see what they're doing now . . .forcing body purchases based on existing lens repositories. . .I only have 2 lenses that REQUIRE a motor (11-16 2.8 and 24 2.8), but I would more than likely go with the D700(s)/800 series anyhow. . .