I know exactly what you want. You want to be able to copy a movie onto your apple tv, delete it from your computer and then keep it on the apple tv - much like you could do with an iPod.
Well, it's entirely possible. I keep all my movies on an external hard drive. Then, I move it to my iTunes library while holding down the option key. This basically tells iTunes to keep the movie in its original location and not copy it to a library. Then, I sync it to my apple tv, making sure that the external drive is plugged in or networked through my airport extreme.
Once that's done, I can unplug the external. As long as I don't attempt to watch the movie in iTunes by accident while not plugged in - and then forget to fix the exclamation mark that tells you the file could not be found - Apple TV won't know that the file is not on my computer.
Hope that helps.