Here's the official announcement, straight from the official site:
There has been much speculation regarding Halo 2's release date since we announced the game last year. The fact is we don't like announcing release dates unless we're certain we'll meet the deadline.
We're still not ready to pick a specific release date, but we do want to state for the record that Halo 2 will not ship in 2003. This game is a vast undertaking, considerably more complex than any of our previous efforts, and there is just no way to squeeze all of the necessary work into this calendar year. We know many of you were hoping to get your hands on Halo 2 during the holiday season, and we're sorry to disappoint you - but making this game as good as we possibly can is our paramount concern. Microsoft understands and supports our decision, and we hope you will too. Our ultimate goal is to make Halo 2 a great game; when the game comes out next year, we think you'll agree we made the right choice.
Of course, you'll be able to get a small taste of the glories we have in store when we show Halo 2 at the Electronic Entertainment Expo in May. And there is Halo for Mac and PC, still on track for a 2003 release with several new features and content. There is lots of exciting stuff on the way for Halo fans. http://www.bungie.net/perlbin/blam.pl?file=/site/1/news/news_detail.html&id=A72CE2BD-