The Nvidia Shield launched in late July 2013 and our review of the system deemed it an "expensive novelty." Nvidia upped the system's PC streaming functions to 1080p and added button-mapping options in December.
While it is unclear whether Portal will make its way to the Google Play store for use on more Android devices, all signs point to its exclusivity on the Nvidia Shield. We've contacted Nvidia to clarify that point, and will update as we learn more.
Update: Nvidia informed Joystiq that Portal is a "native Android game," and will be sold on the Google Play store for use on other Android devices.