Having now sat through the keynote and had a look at the WinRT stuff, I'm unconvinced that "WinRT is Windows" as the presenters claim.
To me, WinRT is a series of native, "Win32" if you will, components that all have suitable managed wrappers that run on the CLR4. WinRT itself is a framework that hosts the CLR4 (rather than having a separate CLR fork) and includes the BCL4 assemblies in addition to the new Windows 8-specific ones (such as the sensor framework).
It explains a lot: the JS stuff most likely uses the DLR - which means it's possible someone will add the ability to use Python and other DLR languages. The presence of C++ was probably referring to CLI/C++ rather than pure native C++ on account of how the "XAML" box overlapped both the C# and C++ blocks.
I believe the source-level compatibility with Trident is a coincidental with the fact the DLR JS runtime strictly complies with the ECMAScript specification. I don't believe the native JS JIT engine makes any appearances in WinRT. This is further evidenced by the support of the HTML+JS platform in Blend, which is just a design-time host for XAML and presumably a similarly .NET-derived HTML+JS platform.
Off-topic, if this means Microsoft has developed a managed HTML+JS platform, it's possible we'll see a decent web-browser made available for the Xbox 360.