They have it working on ARM internally already. And the current WP7 OS kernel is Windows CE, one of the most widely ported OS of all time, running on SH4, MIPS,

They have it working on ARM internally already. And the current WP7 OS kernel is Windows CE, one of the most widely ported OS of all time, running on SH4, MIPS, x86, ARM i.e. almost any little-endian architecture.