Despite mounting speculation, Mark Rein, Vice President of Epic, recently shot down rumours of the sequel on the official Gears of War forums, posting:
"It is complete nonsense. We have not announced a sequel to Gears of War to Gamepro or anyone else for that matter."
While they haven't made any official announcements, we know from one of our insider's at Microsoft, that they are in fact working on the sequel (see Gears of War 2 Release Date Info).
We'll have to wait until the latest issue of GamePro hits the street to see exactly what they've got.