Snow Leopard has a bug, confirmed with 10.6.0-10.6.7, where sometimes if you copy a large number of files from an AFP server share, you will get an error -41 in the Finder when attempting to open them.
This bug is caused by the ACLs on the files/folders not being properly inherited after the copy. Since OS X client doesn’t show ACL details, you can’t easily view this from the Finder.
The fix for this is to remove the ACLs on the copied files/folders. The easiest way to do this is use the shareware utility BatChmod, drag drop the folder containing the files that won’t open, and check the options for “Clear ACLs” and “Apple to enclosed folders and files”.