I came up with this AppleScript tired of Mac OS X's TextEdit app complaining about the file's UTF-8 encoding.
It simply sets the text encoding xattr (extended attribute) of the selected files in your current Finder window. You can save the script, and add it to your Finder's toolbar, or bind it to some key combinations using tools, e.g. FastScripts.
Now you can simply press the button or hotkey and open it again whenever you encounter such errors. Moreover, it lets you QuickLook your UTF-8 text files. :)