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There is speculation (from MSWin) that Microsoft will begin development work on the Windows 8 Beta in the middle of July 2011.
Also, it seems that Microsoft was due to complete a CTP build (Community Technology Preview) version of Windows 8 to show off at it’s Worldwide Partner Conference in Los Angeles on July 10-14. At this point though, that it believed to be unlikely.
Sources do say that they expect Microsoft to begin work on the beta branch of Windows 8 sometime between 14-28 July and that Microsoft will be actively working to to get the beta ready for it’s BUILD conference in September, where it may be distributed to developers.