There is no official word from Microsoft, but speculation points to the second half of 2011, around fall, for the public availability of Windows 8 Beta.

Supposedly, there will be a Windows 8 flavor tailored specifically to emerging form factors, including slates / Tablet PCs, whether they are powered by System on a Chip (ARM architectures) or by traditional processors.

Since Windows 8 is just at M3, there’s still plenty of time for the software giant to build the next generation of Windows in accordance with the evolution of technology and consumer preferences.
Read more at: Windows 8 Should Embrace New Computing Paradigms - Softpedia