According to the site:
While Rockstar did release two major content additions for GTA IV, the company has been very quiet about the future of the franchise, other than sort of admitting that a new installment is in development. The major reason for speculation that a holiday release this year is the goal is due to the fact that contracts of some key personnel responsible for the GTA franchise expire in January 2012, and it would be favorable to their negotiating position to deliver GTA 5 before their contracts end.
Some of our sources say that the lack of any information about Max Payne 3 might also be a clue that resources are being directed toward the development of GTA 5. This seems to agree with what industry analyst, Michael Pachter, believes is the case.
Fudzilla thinks GTA5 will be announced at E3 2011. However, according to analyst Doug Creutz, an actual release will not happen before March 2012.