We're in the same boat. I really wanted a D400 because I really liked my D300 a lot. I decided to get the D600 while waiting for a possible D400 thinking I would just sell the D600 if the D400 proved to be the camera I wanted to replace my D300.
My D300 will be up for auction next week, and I really don't think I care about a D400 anymore after using my D600 for only a week. For me, the D600 is likely going to remain my main camera for years to come.
There are little things I miss about the D300, but not enough to make me wish for something more than the D600 right now.
At any rate, I have a body I like and I'm out there shooting when I can (with two young kids it's not always easy these days) instead of waiting for a new body to come along.