No, this isn't a glitch. Halo: Reach is technically available on Xbox Live Marketplace right now, a full month before its official retail release. But before you load up on Microsoft Points to get early access to the game, be warned. Even with $1250 worth of MS magic money, you won't be able to download the game. Unfortunately, you'll need a special code that unlocks it.
According to an official Microsoft Support document, "customers can see the item in the Web Marketplace but cannot purchase it unless they have the special pre-paid token ... The content is available only to users who have a special 25-character pre-paid token that enables them to download the item."
It's likely that the full version of Halo Reach is being offered to reviewers via digital distribution; however, don't expect Halo: Reach to be available via the Games on Demand service. Microsoft had previously enabled early digital distribution of Crackdown 2 for reviewers -- and Crackdown 2 is still unavailable to the public via Xbox Live.
Update: A Microsoft spokesperson has offered this statement on the matter: "We plan to provide a limited number of copies of Halo: Reach using the Games on Demand technology for media review purposes only. While we are thrilled to be able to take advantage of the technology to provide media with a more accessible and convenient review experience, there are no plans to launch Halo: Reach to the public through Games on Demand."