Ok, I believe I've figured this one out. The function handle_depth_halftone (epson.c) indicates that for threshold to be enabled, the mode must be Binary and halftoning must be None. After adding in some fprintf (to stderr) statements, I discovered that the order of the command line arguements also matters with these options. I seem to remember reading this somewhere, but I must have forgotten. Epson's Image Scan! for Linux 1.4.0 still doesn't work. I'll try their new 1.5.1 release.
I apologize if I caused anyone to waste time on this. Peter
