I isolated the problem some. It occurs when you try to copy a resource file in the format "._<whatever>" from a HFS volume to a CIFS share from within the Finder. To demonstrate:
1) Enable viewing of invisible files (via program MainMenu, or via "defaults write com.apple.Finder AppleShowALlFiles TRUE;killall Finder" in a terminal)
2) Open a the directory that will not copy to the CIFS share, you will find at least one file prefixed with "._", it is a resource fork file.
3) Try to drag just that resource fork file over to the CIFS share, it will produce the infamous "......can't be read or written. (Error code -36)" error.
4) Close the error and try to copy the file again, you will now receive a different error, "The item ..... can't be replaced because it's invisible" !!!
I think that the Finder has a feature where it hides resource fork files on CIFS shares from itself, and probably other non-HFS volumes. When copying such files over to a CIFS share though, it will freak out and produce the -36 error when they suddenly "disappear" after being created.
I spent hours trying various options to work around this issue on the server side Samba 3.0.X and 3.3.X and failed miserably. I think Apple is just going to have to patch the bug.
As an (annoying) work around, I found that you can delete all resource fork files from the directory you are trying to copy. That worked around the problem %100 of the time for me. For example, if the directory "Financial Data" on your desktop won't copy over to a CIFS share, you can do the following:
1) Open a terminal, run: find "Desktop/Financial Data" -name '._*' -delete
2) Close the terminal, copy "Financial Data" over to the CIFS share