Movies have gotten shorter since the days of Ben-Hur and the Ten Commandments, but I don't think our attention spans are to blame. We get bored watching Ben-Hur not due to our own ineptitude, but because it was made for people accustomed to slower thought. There isn't a lot going on in older movies. The same can be seen on a smaller scale in films from the 80's and 90's. Whenever I watch a movie that rocked my socks off as a kid, I'm surprised at how slow and tame it really is.
Movies like Cloud Atlas require the ability to jump rapidly from plot to plot, multitasking and processing previous story lines in the background. If Cloud Atlas was shown to an audience just a few decades ago, I'd wager almost every viewer would be completely lost -- as a minority of viewers are now on movies like Inception, Memento, and Cloud Atlas.
It's a trend in music, movies, video recaps like the one below (see if you can keep up!). The more capable and comfortable we become at processing digital input, the quicker we can learn and experience. But, How fast is too fast and can the trend continue?