Steven Spielberg has yanked the aliens from Close Encounters of the Third Kind out of mothballs, and plans to stick them in Indiana Jones and the Kingdom of the Crystal Skull when Indiana accidentally finds Area 51.
Steven Spielberg and George Lucas have put little inside jokes and sly references to their other films in some of their movies, including a fair share in the Indiana Jones films. In Raiders of the Lost Ark you can see THX-1138 (Lucas' first sci fi film) on the wing of one of the planes, and the opening scene of Temple of Doom takes place in "Club Obi-Wan."
However, Indiana Jones finding aliens in storage at the fabled Area 51? We can hear the faint sounds of a motorcycle revving up, attempting to jump over a shark somewhere. At least this will keep us on our toes when Indiana pulls a lightsaber out of his man-bag and chops E.T. into tiny pieces and then races off in one of the original cars from American Graffiti .