I think that BUILD will actually be about the new compiler technology that Bartok and Pheonix research has led to, and not just about Windows 8. I believe that Microsoft doesn't really have much intention to make tools to directly manipulate HTML/JS, but will extend existing tools, in particular Blend and VS to compile down to HTML/JS - and run on a JS based .NET Framework.
I also think we'll be able to target either CLR=>IL=>JS or CLR=>Bartok/Pheonix=>Native. I think that there will also be a way to compile parts of an application from CLR=>IL=>JS and parts to native. As the Windows 8 leaks have demonstrated, the Windows 8 sample applications have HTML/JS UIs, that interop with the Windows Runtime dlls by marshalling JSON through MSHelpUI.dll to other dlls, which appear to be compiled from C# into native dlls. It also appears that the UI can either be targeted to be output as HTML/JS or XAML/IL, or native Win32.
Of course, I have absolutely no idea if any of this is correct.