While it will support ARM, I'd say it's doubtful that it would be released on a disk that you could just load onto your existing device, it will probably only come preloaded on devices by OEMs.
Originally Posted by Saljen
Not being open source didn't stop the HTC HD2 from recieving Windows Phone 7
Traditionally MS haven't been worried about piracy of their mobile OS's. Their mobile OS efforts have mainly been about keeping people in the Windows eco-system, and to a much lesser degree selling licenses for CE to OEMs making embedded devices. In most cases of mobile OS roms being posted, they've just been updated/enhanced roms for existing Windows mobile devices and so haven't really cost sales and have possibly enhanced the ecosystem.
They're generally much much more concerned about piracy of their main OS. It remains to be seen how they will react to people trying port the ARM version of Windows 8, but they could easily react as strongly as they would for a normal x86 windows.