We've heard before that Apple may be working on a service for iTunes called "Replay," sort of a Netflix Watch Instantly-style service that lets you download and stream any movies or TV shows that you've purchased from iTunes. Obviously, with the iCloud infrastructure in place (and iTunes Match already announced and on the way out), it's not too big a stretch to see that actually happening.
But if such a deal is underway, the latest reports say it won't be available for a while. CNET says it has been poking around film studios about such a deal, and at least six different major studios have yet to sign agreements for a service like this. Apple has indeed been trying to chase down agreements that would allow customers to stream purchased movies any time, but those agreements aren't done deals, and they'll need to be before this thing gets off the ground.
The biggest sticking point seems to be a few older agreements with HBO and streaming service UltraViolet, which allow for some exclusive periods where only those companies can host digital downloads. Apple is probably willing to pay to unlock those periods, but studios aren't in any position to make deals happen fast -- they'd rather see demand (and thus prices) go up. At any rate, if Apple does want to make iTunes Replay happen, it still has at least one tough nut to crack here.