Well today there’s confirmation that Macguire said that over at SciFiWire. (So Matt, who is YOUR source?)
“Well, it’s possible that we make another movie, it all depends on if there’s a story worth telling. …if they come up with a good movie, and the whole team wants to get back together, and we feel like we can make a good movie that’s worth making, then I’m up for it.”
I don’t think we should expect production to start on another as soon as Spider-Man 3 hits theaters, though. I’m betting everyone is feeling the need for a break, and that Raimi has other projects he’d like to work on.