After reading an Xbox tutorial guide by Microsoft, it seems to suggest that the next Xbox might be called the X8 or Xbox Infinity.

The official Xbox website has tutorial pages that explain how to use various features of the Xbox 360. The interesting part of these tutorials is that two pages refer to the X8.

"Play a Game (includes X8)" and "Listen to Music (includes X8)"

This could essentially be interpreted to mean that the instructions are applicable to both the X8 console and the Xbox 360.

Could this be a listing of the compatibility of the X8 mouse with an Xbox platform?

The next-generation console is expected to be revealed later this year, so without confirmation, we're only left to speculate.