It looks like this thing is going to happen, for better or worse.
In an interview with Heeb, Harold Ramis(Egon Spengler) said that the threequel is going to shoot next Summer for a 2011 release...
"Something’s going to happen. Dan [Aykroyd] did write a spec GB3 screenplay a few years ago, but no one was motivated to pursue it. Now, 25 years after the original, there seems to be some willingness to proceed and apparently a substantial public appetite for a sequel. We’ll introduce some new young Ghostbusters, and all the old guys will be in it, too. Think Christopher Lloyd in Back to the Future ... GB3 is progressing with plans to shoot next summer and release in 2011 …"
Until a director is officially attached this will have to join the mountain of rumors for now, but it sounds like we will see this movie at some point. Its possible that Ramis himself could take the reigns, but since his last movie, Year One, tanked, thats probably not likely.

I only watched the first movie on TV today and its still as good as ever, in pretty much every way. Even the slightly dated special effects do hold up surprisingly well. Lets hope that the next movie is more like that..and less like the second one!