Irnbrukid - that is my understanding too i.e. you get a 'type' of build slot ie petrol or diesel and then you can spec as you require once you have the slot.
On this matter, I asked a 'friend' at a main BMW dealership today if I could get a 330d before christmas and he said yes, probably. They are only finalising their slots now for the rest of the year. I asked if it would be eaiser to get a 320d slot and he said it didn't make any difference, you book the slot based on diesel or petrol only.
I have no reason to not believe him...therefore I feel sorry for those that are getting screwed with this 330d deal falling into next year, they must have been able to secure a diesel slot. Why take orders when you can't guarantee delivery - very annoying!!