I've first heard rumors about this last year but it was never made official. And now more information continues to get leaked although this is still classified as a rumor.
I personally would love to see this game remade but not at full retail price. While achievements and leaderboards would be obvious additions, what I'd really want would be online multiplayer(which was missing from the original Xbox due to Xbox Live not being up and running yet). Theres really no reason why they couldn't add it in(it was added to Perfect Dark) but theres still a chance they might hold it back.
The thing that keeps me from believing this is that it would contradict Microsoft's normal holiday gameplan. They have yet to release a new Halo game and a new Gears of War game within the same holiday period(Sept-Dec) and their original plan was to release them both in alternating years. So releasing both Gears of War 3 and Halo:CE remake during the same timeframe could cause them to cancel out some of each others sales and XBL time. But it is Halo's tenth anniversary this year so I expect something Halo to come out before 2012 arrives.