According to Microsoft employee Robert Morgan, future iterations of Windows, including Windows 8, could support a 128bit architecture. Two weeks ago his now-defunct LinkedIn profile said he had been working on "research and development projects including 128bit architecture compatibility with the Windows 8 kernel and Windows 9 project plan".
See also: Microsoft Windows 8 review
Further to this, Morgan's profile also let slip the news that Microsoft is hoping to form a number of future relationships with major players such as IBM, Intel, AMD and others in the run up to 128bit support. (Keep in mind that 64bit computing is only now going mainstream.)
An interview with Robert Morgan, who has been with the company since 2002, is expected to appear on Windows 8 News within the coming days, hopefully shedding more light on what to expect from the next version of Windows, codenamed Chirdori.
So while we wait to learn more on the future of Windows, be sure to check our coverage on Microsoft's latest effort, Windows 7.