An ongoing issue that people have had with the latest 10.6.3 update is with changes to the Finder's handling of files when copying to mounted SMB shares. There have been errors stating the files cannot be copied either because the user does not have permissions to access them, or a Finder error -36 which is a basic I/O error.
These problems appear to be caused by the Finder's handling of extended attributes associated with the files being managed, some of which may be hidden files or split "forked" files. While a full fix will need to come from Apple either updating the Samba implementation or the Finder so they work better together, there are a couple of methods for overcoming these errors and getting the files to transfer.
Clear dot-underscore files
If you are copying a folder, launch the terminal and type "dot_clean" followed by a single space. Then drag and drop the folder you are copying to the Terminal window. This will complete the full path to the folder in the Terminal window, after which you just need to press enter. The command will remove the dot-underscore files and hopefully allow the file copy process to work for that folder.
Zip and transfer
Use the Finder's "Compress" feature (in the contextual menus) on folders you are transferring. This may be a bit cumbersome for some situations, but will turn all the selected files into a single zipped archive which will simplify the copy process and potentially avoid the errors.
Change the Samba settings for unix extensions
Samba has the option to not recognize unix extensions when handling files, which will prevent it from interacting with the extended attributes the Finder appends to files. This was recently reported as a workaround on Slashdot, and here is how to implement it:
First get a copy of "TextWrangler" (free from BareBones software) or another text editor that supports authentication.
Choose "Go to folder" from the Finder's "Go" menu
Enter "/etc" in the field and press enter. WARNING: Be careful in this folder, and only modify the files we mention.
Locate the file "smb.conf" and copy it to the desktop as a backup. Then open it with TextWrangler or your preferred text editor.
Under the [global] section, create a new line and add the following text:
unix extensions = no
If you use TextWrangler, upon editing you will be asked to authenticate, in which case supply your administrator credentials.
Save and close the file, then either restart the system or toggle file sharing off and on again in the System Preferences.
Use the Terminal
If you are savvy with the Terminal, try using it to copy files to your mounted SMB shares with the basic file handling commands (mv, cp, etc). The problem at hand appears to happen only with the Finder, so copying methods that do not use the Finder should work.