I've rented a movie using my MBP15, transferred it to my iPod touch, then back again to the MBP15. This was all before I started to watch it.
If memory serves, there is an area under the Movie tab were you can move it.
I don't believe that you can move the movie once you've started watching it.
Edit-1: Downloading a rental movie right now. After DL, I will try and report back.
Edit-2: After you DL the rental movie, connect your iPod touch. Then go to the Movie tab. At the top of the page will be the rental movies. Select which one(s) you want to transfer. When you transfer the rental movie, the rental movie is actually copied/moved. After you transfer, you can transfer back if you want as long as you haven't started playing the movie.