from the halo faq
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It's important to understand two things: this decision is left to Bungie's development teams, and in Halo's case it has not been made yet. The development team has a ton of options to consider, and thats what theyre doing right now. They may choose to concentrate on one platform, to bring the game to every platform under the sun, or something in between. As of this writing we simply dont know what the answer will be. We cannot promise that a Windows or Mac version of Halo will ship, but we can't rule it out either. Nor is there any reason to at this point.
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chances are though that a mac version of halo will be released in due course, imo