According to IGN, Dan Aykroyd was a guest Wednesday night on the radio program Coast to Coast AM with George Noory on L.A.'s KFI AM 640 where the actor gave a brief update on Ghostbusters 3.
Aykroyd advised Noury that screenwriters Gene Stupnitsky and Lee Eisenberg are currently "hacking out a first draft" and that Sony is enthusiastic about the project. Although he gave the impression that Ghostbusters 3 was far from a go project, Aykroyd did say that for the first time in years it seems closer to a reality than it's ever been and that both the cast and studio are all eager to finally make it happen.
Aykroyd didn't divulge anything further on the project than that, but his cautious tone made it seem like it wasn't as much of a certainty as it was before the summer release of Harold Ramis' flop Year One (also scripted by Stupnitsky & Eisenberg) when Aykroyd, Ramis and even Bill Murray were all busy talking it up. Aykroyd even suggested this past spring that GB3 could begin filming by the end of this year.
Thanks to IGN for the info.