This caught me out last night while playing with a slaved flash, I couldn’t get the auto focus assist light to come on my D300
So I did a search this morning, and found http://photo.net/nikon-camera-forum/00OikF
“AF-assist illuminatior is only available when both of the following conditions are met:
1. Illuminator is turned on (pg.273)
2. Single-servo AF is selected for focus mode (pg.62).
3. Auto-area AF is chosen for AF-area mode, or single-point or dynamic-area AF is chosen (pg.64).
4. Center focus point is selected. I mean the focus point selection must be in center of viewfinder(pg.66)
I knew about 1,2 and 3. What caught me out was 4