It's a pattern with all Rockstar's titles. People think they're going through some developmental hell or something, but they're really not because they want to give the spotlight to only one title at a time. People wondered about Red Dead Redemption, and to let people know they were still working on it, they released some small footage in one of the GTA IV DLC trailers. Same with L.A. Noire, they started talking about it after they've gotten the Red Dead Redemption DLC out of the way. And with Max Payne 3, they also announced GTA V during Max Payne 3's marketing.
I'm sure they're still working on it, they just don't want it to give it a spotlight because we're in the middle of GTA V's release. They'll probably announce DLC for GTA V going through the course of this year, and then Agent marketing maybe next Fall. Who knows after that.