News : 2005 : Anticipating Spider-Man 3
Sure it's a long way off, but the official site Spider-Man 3 is already up and running at Sony. Official release date is May 4 2007. Sony do list Topher Grace (Eric from "That 70's Show") as confirmed for the cast, but they refuse to confirm the "Sandman as Villain" rumour.
For unofficial sites, you can always try SuperHeroHype, though currently all they have is a repeat of the official Sony press release. There's not much at IMDB yet. At eFavata they mention the Sandman rumour, but little more.
We'll keep our ears to the ground, and let you know as we hear new info.