Already here:

http://www.youtube.com/watch?v=ItkWCmkxBv0

However, I would wait at least for Cortex A15 chips to arrive, especially if you're going to want to use Unity, which isn't exactly a race horse on PC's either. I'm sure Canonical will port Ubuntu to the upcoming Nexus 10, too.

Surely by now somebody's managed to get Linux on a Surface RT? It sounds from the review like the hardware is pretty good, just the software that's lacking.