Ah, I see. I assume that both you and the client use Macintosh computers -- what version of Quark are you using, and what version of Quark is the client using?
If the PhotoShop files are showing up as QuickTime files, that leads me to believe that the zipping process is somehow stripping the files of their resource forks/metadata (the stuff that Mac OS X/Mac OS 9 uses to identify files and associate them with their respective programs).
Does the client use StuffIt, and, if so, have you tried the same process with the client "stuffing" the files rather than "zipping" them?
Also, when you say that the Quark files are corrupt -- do you mean that Quark, upon opening the offending files, says something like, "This document appears to be damaged" or does something different happen -- perhaps, images and text are shifted on the layout with no error message being thrown up?
I know there's a lot of questions and stuff, but I worked in digital pre-press for 8 years and have seen/troubleshot my fair share of corrupt documents and cross-platform incompatibilities... we'll get to the bottom of this!