To my knowledge, RT will be preinstalled and not 'sold' as an OS. (Android, in comparison, allows one to simply download the Android source and build the OS as they see fit.) Dedicated hardware will come with RT, and Microsoft will release patches and updates along the way.
That said, Android has been ported in limited scope to the R-Pi, so who knows. It's possible, but it'd likely run extremely slowly given that the Pi is a glorified Pentium 2 with HD graphics - hardcore processing isn't exactly its specialty, you know? You're better off with one of the linux derivatives. Given that RT is ARM based, it'd probably run, but good luck getting a working image of the OS. And then you've got the whole touch aspect to deal with at that point, too.