I think the D600 was a replacement for the D700 not the D300.
There will be no reason to hold back on the new top of the line DX camera. Expect faster, bigger buffer, more resolution and higher ISO performance. Speculating further, there may even be two new DX cameras in the Spring of 2012. An upgrade to the D5100 (with articulating screen), the D7000 and/or D300.