Internet rumor has it that Bruce Campbell will take a villainous role.
Warning: Spoilers
Internet blog site Film Ick says that Spider-Man regular Bruce Campbell will play Quentin Beck (Mysterio) in Spider-Man 3; the fourth villain in the film along with Sandman, Venom and Hobgoblin.
Earlier predictions for a fourth villain mentioned either the Lizard or Black Cat. However, Campbell and his buddy, director Sam Raimi, have always said one day he'd be upgraded to villain.
Film Ick says: "The extra villain that everybody has been speculating about makes only a fleeting appearance, and then only in a very off-hand but amusing way. Bruce Campbell's character this time around is Quentin Beck - known to fans of the comic as Mysterio. Again, don't take this as set-up for a subsequent film, but if anybody ever does put Mysterio in a film and NOT cast Bruce Campbell, they'll have hell to pay."
Wikipedia lists this description of the foe:
"Quentin Beck was an expert designer of special effects devices and stage illusions, a master hypnotist and prestidigitator, and an amateur chemist and roboticist. He had extensive knowledge of hand-to-hand combat techniques learned as a stuntman but didn't otherwise possess superhuman abilities. His personal weaponry included a one-way plexiglass helmet with holographic projector and gloves and boots armed with nozzles which emit hallucinogenic gas. He did have some more advanced technology (not of his own design) at his disposal."
Look for Spider-Man 3 May 4th, 2007.
You can read more spoilers at Film Ick.