I have run accross the same exact problem.
Which webserver are you using?
At work I am stuck using IIS / win2k [sucks for me, I know].
I have not been able to successfully access ANYTHING on a network drive via a php script. No directory listings, writing files, zilch.
I believe this is because the system user that the IIS / PHP script runs as, does not have network privlidges.
Maybe if there was a way to make IIS run under a different user [like a domain user with rights to the remote folder] it would work.