According to this Cliffy B confirmed GOW2 in June 06
Gears of War 2 Coming
June 19, 2006
Gears of War lead designer Cliffy Bleszinski, or Cliffy B as many call him, recently said that as soon as Gears of War is finished, he’s starting on a sequel. Cliffy B was talking with Yahoo! News, where he let in on what he’ll be doing in the future, after Gears of War. “Gears of War 2, I’d imagine” Cliffy said.
Seems like Epic is confident that Gears of War will be a big hit, the game has already been hyped up as one of the most anticipated games coming to the Xbox360. Right after Halo 3, of course.