I've said this before and I'll say it again: Microsoft isn't doing this to ban Ubuntu / Linux in general, but to prevent Android from spreading. With ICS being a great OS, many (tech) folks would simply dual-boot ICS on their WinRT devices.
I don't see the point in these decisions to expend likely a lot of time, effort, and money eliminating edge cases.
Not that it would be likely, but I'm sure Microsoft would want to stave off any chance of that happening in the early days of the platform.
It looks like ARM has gotten some virtualization extensions recently, but I still have my doubts as to how pleasant the experience would be on sub 1.5GHz processors.