Cliffy B had announced that Gears Of War 3 would be released on April 8th 2011 but that wasn't something Microsoft agreed with so the date is now pushed back another 5 months.
Microsoft has stated that it is not due to an issue with development but rather a marketing plan, with that being said you can be sure Microsoft is looking for maximum profit with their game studio. Epic has their game Bulletstorm coming out in February and if they released gears too early it might take away from the popularity of that game. After watching the trailer for it I made my decision that it's a game I won't be buying, mostly because it looks like it could entertain me for a total of 2 weeks and then never touch it again.
I have to say in one sense it's smart to push it back for money and it gives for time for testing but in the same sense I'd rather have it earlier.