Possible announcement during late summer (August).
Apparently, according to Official Xbox Magazine, Rockstar may be announcing GTA V sometime during August. There is a Gamescom Convention in Germany around the same time.
It would make sense if they did announce it next month, they announced GTA IV just over a year in advance to its release. It has been said that a 2012 release is more than likely. If they announce it next month, it will make an 8-9 month stretch till April (April 29th 2008, GTA IVs release).
Possible? Could be, but who knows...