The "on files added" Folder Actions handler in 10.6 now waits for a file to finish "arriving" before being called. Earlier versions would call the handler as soon as the a new file hit the folder. But if the file was very large the handler might fail because the file hadn't completely arrived in the folder. Now, under 10.6, if the file-being-added stays the same size for more than three seconds, it is deemed "done", and the action is called.
The workaround suggested in this Folder Actions and iTunes tip is no longer necessary in Snow Leopard.