Anyway, this question was featured on 'Ask the Jedi Council' at the official site a couple of years ago. That entire feature seems to have disappeared from the site, but here's the actual quote:
Quote
Why haven't we seen the stars stretch into lines when a ship jumps to hyperspace in the prequel trilogy?
For the definitive answer, we turn to none other than George Lucas himself. Here's what he had to say:
"I think of the series as one movie. In order to keep the impact of the hyperspace jump in Episode IV, I have decided not to use it in the first three films. If you're watching them from one to six, you'll get the same thrill."