As Fusible points out, there are several indications that Xbox Fusion could serve as the official name. For starters, Microsoft owns more than just the domain's dot-com; it also owns a slew of other domains like XboxFusion.info, XboxFusion.biz,XboxFusion.de, XboxFusion.co.uk, and more.
However, domain names may not only serve as indications as to what a product could be named, but also what it won't. In the case of the next Xbox, we can probably rule out the Xbox 720, Xbox Infinity, Xbox 8, and Xbox Durango. Although these are the most popular of the rumored names, Microsoft's lack of domain name commitment to them suggest these won't be the official names.