From what I have heard, if you touch the file at all it locks itself.
It's a copyright thing. Just like if you take the SD card out and put it in your computer, the movie will no longer play either. The 2GB card put in the phone from the factory was more or less just for the Avatar movie. But you can't play it or use it anywhere else. You can take the factory card out with the movie on it and replace it again and watch the movie so long as you never tried to put the SD card into a computer or other device. My guess is that by moving the file, you have locked it and you can longer watch the movie unless you take it back to T-mobile, maybe they can do something about it?