Valve's Portal and Half-Life 2, despite being quite old in terms of PC games, are two of the best showcase titles for the SHIELD right now. Naturally NVIDIA (which publishes both games in the Play Store) is eager to highlight them for the shiny new SHIELD Tablet. Actually, it's not shiny - the tablet is quite matte. But anyway, both games have been updated with SHIELD Tablet support, a week before the device is released.
The app update text doesn't mention any specific changes to gameplay or graphics, but hopefully they include on-screen controls for people who buy the SHIELD Tablet without also purchasing the $60 SHIELD controller. NVIDIA representatives told us that Portal and Half-Life 2 will have at least some graphical improvements on the tablet that take advantage of the more powerful K1, above and beyond the 1080p resolution bump.
Sadly, both games are still exclusives for NVIDIA's first-party SHIELD hardware - not even the Tegra 4-powered Tegra Note tablets have access to them. That's unlikely to change any time soon.