It was Tegra 2 @ 1Ghz with 1GB of RAM used in MIX '11 showcasing Windows 8 with latest Internet Explorer (IE) version. After that Microsoft moved to the machines running Tegra 3, possible with 2GB of RAM. Tegra 3 is extremely nice CPU with 5 cores. If you would check Tegra 3 benchmarks, it has more raw power than INTEL C2D @ 2Ghz. Both, Tegra 2 and Tegra 3, are using ARM Cortex-A9 cores inside.
Microsoft will support ARM SoC from Qualcomm (Snapdragon), TI (OMAP4 and OMAP5 probably), and NVIDIA (Tegra 2 and Tegra 3).
This is mostly done to bring Windows 8 to tablet/slates market. Let's hope Windows 8 will be supported on Trim-Slice (Dekstop nettop based on Tegra 2) and they will move to Tegra 3 this year or early next year.
We already can see next gen ARM SoC coming, i.e., Tegra 4 (Wayne) and OMAP5. Those SoC will have ARM Cortex-A15 inside. Way more power than the current cores used.
Don't expect R-P supporting Windows 8.