Would it not make sense for the Valve Steambox to run on ARM since they're cooperating with Nvidia (and just launched Steam for Linux?) I thought the Nvidia Tegra 3 was considered quite the powerful chip.
Language doesn't matter so much as the API - you can write against DirectX for both the Xbox 360 and the PC, making appropriate changes. "Porting" to/from the [for example] PS3 is a much bigger ordeal.