Nikon produced a number of different SB-800 manuals over the course of production. The most accurate one has the answer: AF Assist only works with 24-105mm lenses (FX). With wider lenses the patterns in the projected red bars are too small to get seen by AF sensors in the camera correctly.
But this brings up another good point: AF Assist from the flash (all models) is highly variable in which focus sensors can be used. This varies from flash to flash, focal length to focal length, and yes camera to camera. It makes for a really ugly table. Just make sure you know which autofocus sensors your camera and flash unit support at your common focal lengths. It's in the manuals if you look hard enough. It's also easily found in my Complete Guides for the various cameras.
