New documents from the Steamworks Documentation site show that Steam Cloud Play (Beta) is now being implemented.
The new name of the service is Steam Cloud Gaming, and for it to be enabled, each game must support Steam Cloud Saves.
In addition, each game developer has a choice to opt-in: “Enable your game to run streamed from the Cloud, hosted by Valve, and the following service providers:” and “NVIDIA GeForce NOW.”
Nvidia's GeForce NOW has been running for quite some time as a PC gaming streaming service, and the real news is that Valve is implementing their own Valve-hosted game streaming service.
It appears from the wording of the opt-in that Valve's own-hosted game streaming service is a prerequisite to enabling native GeForce NOW support.
It remains to be seen exactly how game streaming will look and function within the Steam client, and we look forward to seeing full implementation soon.