According to the MS Nerd, the next-gen console, codenamed "Loop", will feature a version of Windows 9 (apparently skipping the already announced Windows 8) and ARM-based processors. Even more important to gamers, the "Xbox 720" will be "far smaller" and less expensive.
The blog goes on to say, “With a heady mix of rumors, tips and speculation, I am now stating that Xbox codename ‘loop’ (the erstwhile XboxTV) will indeed debut a modified Win9 core. It will use a Zune HD-like hardware platform-a ‘main’ processor with multiple dedicated assistive cores for graphics, AI, physics, sound, networking, encryption and sensors. It will be custom designed by Microsoft and two partners based on the ARM architecture.”
Tons of rumors have been coming out in recent months regarding Microsoft's next console, including resumes listing work on a Microsoft next-gen project.