Don't get too excited yet, as the information on this is pretty scant, but if a tweet from Production Weekly is correct, Sony Pictures 'is planning to put Ivan Reitman's “Ghostbusters 3” into production in May 2011'. However they do add that they're just 'hearing that', rather than it being definite news, so don't break out the party poppers just yet.
Sony has been working on the film for quite a while now, with The Office and Year One writers Lee Eisenberg and Gene Stupnitsky coming up with a script, which has more recently been refined by Dan Aykroyd and Harold Ramis. However there are still a lot of hurdles to jump. Deals made back in the 80s for Ghostbusters 2 mean that Aykroyd, Ramis, Bill Murray and director Ivan Reitman all have an absolute vito on any further sequels, so each of them need to agree for the movie to go forward. Sony may well be put plans in place to shoot next May, but that's provisional on everyone agreeing to it.
There was a very positive sign recently, as while Bill Murray has been a bit curmudgeonly about the idea and seemed all ready to be a roadblock, he recently showed up to Spike TV's Scream Awards wearing his Ghostbuster gear, suggesting he's warming to the idea.
Although no official plot details have been released, rumours suggest Murray's Peter Venkman could be a spook himself in Ghostbusters III, with much of the plot revolving around his now grown son, Oscar, who's planning to put on the ghostbusters gear, and who still has issues with his dad despite that fact Peter is now protoplasm.