SteamOS for Apple Macs is finally here: AAA PC games get tested on MacBook Pro
(www.notebookcheck.net)
Yeah, compatibility is still (currently) an issue with it.
Horizon, for example, won't load because it checks the GPU prior to launch, finds it lacking, and throws up a message. Crossover, however, has no such trouble.
So I guess as ARM gaming on Steam matures, so too will this.