As late as 1995, George Lucas was planning to film all three episodes of the Prequel Trilogy back to back to back. For many reasons this proved to be an impossible endeavor, LucasFilm lacked the time and special-effects know how to film all three films at the same time. In fact, the digital-animation technology needed for the post-production of Episode 2 and 3 are still years away from perfection by ILM.
Also, George had planned to release the films in two year intervals (1999, 2001 and 2003). But this also proved to be unrealistic because as George Lucas told the Star Wars Insider (1-800-TRUE-FAN), it takes more than two years of post-production work to complete a Star Wars film. It would be very difficult, if not impossible, for Lucasfilm to release these films in two year intervals. Hence, Lucasfilm will be releasing the films in three year intervals.
The current release dates for the Prequel Trilogy in North America are:
Episode 1: Friday, May 21, 1999 [This release date was recently posted on starwars.com. Ep 1 will be a film release focused on quality over quantity. Perhaps as few as 2,000 theaters will receive a print of Ep 1. It is now very unlikely that non-digital screening rooms will be given a print of Ep 1. If your local theater doesn't have at least one digital screening room, you better start begging them now to upgrade immediately!]
Episode 2: May 2002
Episode 3: May 2005