Most of the networks I've setup that had Samba file servers were small and didn't have domain controllers. So there would be a preston:password entry on the server and on my desktop and every time I'd request a resource from the server by virtue of the fact that I'd logged in with preston:password to my desktop, the server would receive those same credentials, see I was good and give me permission to read/write, etc. I think what's happening here is that I'm not authenticating against the domain. My machine is in the domain now. I can mount the shares. But once the shares are mounted and I try to write, for example, I'm not passing along to the domain controller that I'm preston and here's my password.
Hopefully that makes sense.
Preston
What version of Samba are you running? Bugzilla is showing a heap of W2K3 bugs with some versions of samba.
From just searching and reading, I'm getting the feeling you're not actually doing anything wrong, but that it is a Samba issue, not yours.
Regards, Ed.