Microsoft owns domain names related to "Xbox Fusion", according to WhoIS and Fusible, yet do not own any domain names related to other speculated titles of the next-gen Xbox.
Microsoft has owned domains related to Xbox Fusion for years, such as Xboxfusion.com, Xboxfusion.info, Xboxfusion.biz, Xboxfusion.co.uk, and Xboxfusion.biz.
Other names we've heard rumored for the next Xbox is Xbox Infinite, Xbox 720, Xbox Infinity, Xbox 8, Xbox and Xbox Durango. Microsoft does not own any of the dot com's for THOSE names.
It's important to note that the infinity logo we are seeing floating around the net is also a symbol that is interpretated as "fusion".
So, if I were a betting man, I'd place all of my chips on Xbox Fusion being the name of the next generation Xbox. Unfortunately, we will have to wait until May 21st to see if I'd win a fortune.