Can't understand why it's not the D400? So to my mind it wouldn't be a progression from the D300 but a new target audience!
I'm guessing it's because it's an FX rather than a DX. This way the D300 will eventually be surpassed by the D400, and the D700 will be surpassed by the D800.
I'm more curious about what will replace the D60, seeing as there's already been a D70.