This is likely referencing Windows 8, and Microsoft are just being overly cautious in snapping up new web addresses. However the '8' could also represent infinity for next gen.
Rumours abound that the next Xbox, codenamed Durango, will get the moniker Xbox Infinity. Other domains seized in the name of capitalism are XboxPhone.com and XboxTablet.com.
There's also XboxLiveTV.com, XboxCompanion.com and then Xbox8.us and Xbox8.org. A version of Windows 8 is said to be implemented in the next Xbox console, allowing them to feature their fancy SmartGlass and other multiscreen sharing craziness. Other names for the next Xbox are 'Xbox 720' and 'Xbox Loop'.
Microsoft is currently battling for domains Xboxliverewards.com, Xboxsports.com, Xboxsmartglass.com and Xboxmusic.com. There's been no move against Xbox8.com though, owned by a Chinese national.