Different parts of Star Wars were shot in different areas. THe original moview as in Africa but for Return of the Jedi they used Buttercup - Dwayne ( the ASA Membership Director) actually came across them durring filming and has a few of his own photos of the sets....one of these days I'll get him to scan them in
The reshoots for the special editions of Star Wars were also shot at Buttercup. I've got a couple of local friends who went out there and saw some of it.
Scorpion King's dunes are Buttercup as well - one of my friends got to serve The Rock Dinner after filming one night....he polished off two porterhouse dinners by himself I'm told :shock:
Most dune filming is done in the summer so it's a pain for the actors but less of a chance of the public interfering.
filmyuma.com has a nice list of movies that have been filmed down here. I think there's a site for an El Centro film comission as well with more dune info but I can't remember what it is right now.