That's the path I am taking. The D700 will "only" be my second DSLR. I put "only" in quotes, because I would love to have had more, but I really do think it is a bit crazy to upgrade camera bodies with every new generation of advancements. When it comes to making pictures, people can create far better images than I with a D100, D1, or manual film camera. I think that I have successfully fended off NAS and tried to stay prudent about upgrading my camera body. I knew full frame would be here eventually.
I bought my D70 when Nikon introduced a digital camera that could do everything that my N80 film camera could do at around $1,000. Since then, there have been many opportunities for upgrades. I held steady when the D200, D80 and even D300 came out (although I almost bought a D300 a short while ago).
In light of recent advancements, my 1.8" LCD and 2.5 frames per second are getting old. My tiny little view finder is a pain. I have limited image adjustments and settings in my menus. Not being able to go over ISO 800 without horrible results is quite limiting (although still better than film days).
Now, the D700 offers all of the great advancements in the D300 that I coveted, with better ISO performance and no cropping. I plan to add some nice Nikon primes and I will be all set.