I also think that we'll see this game in stores between April of and October 2012.
Rockstar, when it comes to marketing GTA games, tend to err on a tight adverting/pr ship. Vice City was revealed in July 2002, came out October 2002. San Andreas was revealed in May of 2004, and released October 2004. GTAIV was announced in 2007, and was supposed to have a 7 month marketing campaign until release but was delayed, but it still it put just a year shy of its initial reveal.
Whenever Rockstar unveils a new GTA game, it's coming soon and they make a huge deal out of it because they have to get their PR machines a turnin'.