The FX format refers to 35mm format, meaning old style lenses (of which I have a few) will be fully compatible. Previously Nikon only offered the top of the line D3 as an FX format digital camera. Now it has a smaller, less expensive sibling. A useful first review (not really a review, more a list and comparison of features) can be found here.
A couple of comments:
First the price is expected to be $2,999. That's $3,000 in real money, plus tax, of course. A lot more than the D300 but a lot less than the D3. I won't be replacing my D200 for a while!
Second, the numbering scheme. we have had a D100, D200 and D300. Suddenly a jump to D700. This implies there is some life in the DX format. But the logical conclusion still must be that FX will ultimately win over DX, so any major investment in glass should be aimed at the FX range of lenses.
See an earlier Focalplane discussion on the D3 FX format here.