I'm already on the wait list for a D600. I have a D800E.
My thought process is I will convert the D600 to an IR camera rather than convert an existing D300 or D7000. For my IR work, my most frequent subjects are landscapes, architecture, and street. An FX body fits those subjects better than a DX body. The D600 offers newer technology and larger files than the D700, D300, and D7000. It also offers Live View which is an improvement over my D200IR.
This is a bit of a specialty approach, but the opportunity to have an FX IR body with large files is a major step forward. I'll continue to use my D800E as my primary camera.