Dan Aykroyd, star of the first two Ghostbusters films (as well as The Blues Brothers, which should never have had a sequel), confirmed in an interview with a Chicago radio show that the original cast had accepted the call for a third film.
He confirmed that they are casting and open to finding some fresh faces, adding that both Bill Hader and Anna Faris have been mentioned as "strong possibilities", although it's still too early to call. Aykroyd, who plays Dr. Raymond Stantz in the franchise, also mentioned Eliza Dushku's name again, also confirming (what we already knew) that both Bill Murray and Sigourney Weaver are returning. Columbia Pictures is aiming at a May 2011 start date. [BD Horror News]
You don't need to know anymore than that, although I'm still not 100% sure, since Bill Murray told me something about how he's too busy golfing to even get together with Harold Ramis about the film. But it was always about the script for him anyway, so maybe he finally read it and gave the thumbs-up.
In any case, very promising news…