i found this out earlier today, some of you may already know how to do this but this is for those who do not
1) download Dtunes from Cydia. This comes with something else called terminal, just leave that on your springboard or this wont work.
2) download from cydia an app called 'MobileTheatre'
3) go on MobileTheatre and go to movies (or TV shows, this works for both) and select a movie.
4) click download on the next screen after putting in the anti-bot code thing.
5) now you have to wait 45 seconds before you can use the free downloader; once its ready, tap download.
6) another white screen will come up with two options and some red writing (ignore that it doesnt mean much). click download not stream as stream will not save it.
7) wait for it to download, this could take a while as it is a full movie of course.
8 ) once it is done go to dTunes on your springboard and it will be there
9) (common sense really) it takes up a lot of memory, about 500mb is the most i have downloaded, so once you have watched it a couple of times delete it by going to edit in dTunes then dragging to trash. you can always re-download it if you need it again.
enjoy and remember the quality may not be the best but hey its free so whatever