actually,,you can have some fun,,, by turning off the DX mode, and you can use DX lens in FX mode.. So... why ? well,, you can use a sigma 10-20 lens,, in FX mode..
At 10 mm,, you will see a circle,, but,,,, at 14 mm or so,, the circle covers the whole FX sensor!
So,, your DX lens becomes a 15-20 FX wide angle ! how cool is that?