Embedded below is an interesting 6 minute interview with Steven Spielberg in which he explains at length how the plot of the film evolved. During discussions between he and George Lucas, it apparently moved from a more direct homage to the B-grade UFO movies of the 1950s (originally to be titled "Indiana Jones and the Saucer Men"), to the final film in which the aliens were de-emphasized at Speilberg's insistence, and were "inter-dimensional" rather than "extraterrestrial" in nature (a distinction I didn't really pick up on when I saw it in the theater).
Will there ever be another Indiana Jones movie? Speaking to Le Figaro just a few weeks ago, Harrison Ford reportedly said that, "the story for the new Indiana Jones is in the process of taking form. Steven Spielberg, George Lucas and myself are agreed on what the fifth adventure will concern, and George is actively at work. If the script is good, I'll be very happy to put the costume on again."