I ran into this same failure mode attempting to use a coolmax NAS CN390. I finally gave up on getting the NAS to work, it would never allow me to log on using any of my Mac computers (all snow leopard), I solved the problem by using one of my old IBM Pcs which was running Windows XP Pro. I am able to sign on to that system and store and share files and my printer, however, I have to keep another box running in the background on my network. It appears that only a NAS running Apples File control will work with Snow Leopard. SMB will work but only over a machine which is actually running Windows. Must be some scaled down version of SMB which is burned into the ROM on the NAS single drive servers which relies on components installed with windows or the older version of Leopard in order to operate correctly.
This is to fix the issue when CP works, but Finder copy doesn't.
Why this (I think) is why it happens... Finder in all its crappy glory is to blame, and I think we all new this. The reason why is related once again to the .Files it creates. When you drag a file (say MyFile to copy into an NFS mounted share, finder will create two files ._MyFile and MyFile. What happens is one gets created as the root user (._MyFile) and the other as the actual user (MyFile), so when it tries to write the file, it will fail because the temporary file is permissioned to root.
How to fix it. Well luckily this was happening to me on my Xserve so I just decided enable the root account and use that from then on out. I wanted to do this anyways because i am used to using root for my entire environment. (Please no BS about running as root for security reasons, I'm fine, trust me)
What it really comes down to is how you use all_squash, norootsquash, and other mounting options between your OSX and NFS server. Really you just gotta find a combination that makes root(0) and user(501) act the same to the NFS server. I never had time to figure out correctly the combination, because i just got a free pass by logging in as root and using the norootsquash option on my server, but i'm sure someone might be able to comment on it.