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.