The best app I've seen for this is HBO Go. It has a huge back catalog of HBO content and a revolving set of movies. It pairs great with an iPad. But, in order to get it, you still need a cable or satellite subscription (and HBO).
Showtime has started doing this thing where you logon to their website while the show is premiering to do/see something but I've never done it because I DVR Showtime shows. Showtime had a press release that they were going to launch their own version of HBO Go but I've not see anything.
I'm excited. I think premium channels like this are going to be the change.