This wouldn't guarantee playing anything and everything your Steam Machine could handle. Only "supported titles" would work right out of the gate. You'd have to toggle a setting to enable Steam Play for other games, and you'd receive a warning that the tool might prompt "crashes" and break save files.

There's no guarantee that Valve will roll out this feature in the first place, let alone the near future. If this is emulation, you also wouldn't get stellar performance barring some surprise technology that minimizes the usual processing overhead that comes from running Windows apps in Linux. This does reinforce Valve's claim that it's "still working hard" on Steam OS, however, and it suggests that you might not have to give up quite so many games just to play on a Linux-powered box.