Here's the official statement on the matter passed around to press:
"Gears of War 3 promises to be the biggest entertainment launch of 2011. The teams at Microsoft Game Studios and Epic Games have done great work thus far readying the title for release in the spring of 2011. However, we’ve elected to move the launch of Gears of War 3 until fall 2011 to make it the marquee title for the holiday season."
So, there you have it. Microsoft wants Gears 3 to be the Microsoft-published game of 2011, and there's no better time to capitalize on the even more massive amount of marketing likely to accompany this decision than a holiday release. Word on the street is that people like to give games to other people as presents around this time, so sales tend to skyrocket with a good push.
In a statement following the aforementioned statement (dual statements!) on the official Gears website, Epic Games executive producer Rod Fergusson reiterated that this delay was a result of a business decision. He also teased that you'll be hearing much more about the game's rather promising multiplayer component--which includes four-player co-op, and the new Beast mode which pits man-as-beast against man--in the near future as Epic opens up its HQ for a press visit next week.
"I know fans are going to be disappointed with the new release date, but rest assured that this is a business decision between partners, and not a quality issue. Gears 3 was on track to be the most polished Gears of War game ever when it released in April, and this additional time allows us to polish it that much more before it hits shelves next fall. Next week we have a major multiplayer press visit at Epic HQ, and I'm really looking forward to the fans seeing and hearing about Gears 3 versus multiplayer for the first time. Lots more exciting news to come."