I shot a wedding this past weekend and just for grins I set one of my speedlites on a stand set as slave. I set everything up and it was all firing properly (besides the awful range without using radio poppers) but my AF assist beam was really struggling to help out my AF. I tried all of my lenses with this situation on my 5D and 1D and the only time it didn't hunt was when I would temporarily set my master flash to normal (non-master) mode.
This may be a dumb question and I may be completely overlooking something, so don't be gentle with me! John