Digital BW, The Print

This is probably not the cause, but it's worth ruling out-

you aren't by chance using photoshop 7 and srgB as the conversion 
space (as opposed to Adobe98, say). Photoshop 7 converts the file 
differently then PS 6, but not by very much- I have one very dark 
picture that is posterized by using PS 7 vs PS6.
Jim H.