If i am understanding things right using a DX lens on the D700 gives you a 6mp sensor.
So whilst it is a cross platform DX FX format this camera and the D3 really are designed for use with FX it looks as though not to alienate those with DX lens investment they have added a "reverse" compatability.
Given that the sensor is best that Nikon have I would still doubt that the a 6mp on the FX sensor at any iso be better than a 12.1mp off the D300. I just can't see it.
I was seriously considering the D700 it is pushing my budgets and if I coud be confident of no loss of quality with my DX lens i would go for it. To buy this body and then find to get the best from it to have to invest in new expensive glass would end in me only seeing the kids at the weekend or the occaisonal overnight no way my other half would let me away with this type of spend.
Unlike film bodies which are really almost timeless digital bodies have jumped every two years with significant improvement so again to get a body that in two years may be old technology is difficult to justify.
FX is obviously the future but i think there will be a place for both though think that there will be less releases of DX lens.
So to embark on new glass and new body not right now for me.
Having been excited and sure i was going to get the D700 the now cheap D300 is a bargain!
If you want high ISO have or are prepared to invest in new FX glass the D700 is for you.
I will be really interested to hear about the 6mp image quality on the D700 as i say it again how can it be better than the 12.1 on the D300.