There have been several movies which have been released for online viewing (at a price) at the same time it is released in the theaters (examples: Bubble 2006 and Upstream Color - a month after theater release). I don't think this will ever become a regular practice in the future ... more of an oddity. The reason I say this is, as long as there are movie theaters, theaters will want their cut (theaters do not make their money from the movie itself, but from the concessions they sell - popcorn & soda - at least here in the States). If people are home enjoying movies, they aren't buying concessions.
Another reason is, most main stream movies were meant to be seen on the "big screen". If they weren't meant to be seen there, they'd be shown on network television.
While it may seem releasing movies at home at the same time as in theaters would provide a way for studios to make more money, I would argue it wouldn't exactly work this way. While their upfront money would be greater (make money quicker), the total quantity of money made would less. They release a movie in the theater (money made), then out on On-Demand, then on DVD, on premium cable channels and direct stream, and finally (usually) much later on commercial TV. Here is five different money streams where money is made for a movie. If you put any of them together, they compete for the same pool of money. If I have a choice to view a movie at home vs. at the theater I would probably choose home because it is much more fiscally responsible (multiple people can watch a movie at home much cheaper than what it costs for tickets to go to the theater). Some people might choose to go to the theater, but not nearly as many.
Mind you, it would be great for us (the viewer) if movies were released at the same time to different markets, but given the amount of money which can be made through the various sources, I doubt it will happen anytime in the near future.
Here is an article from the Huffington Post on the subject.