Found this on the shield's FAQ page:

How does streaming games from my PC to SHIELD work?

NVIDIA uses the H.264 encoder built into GeForce GTX 650 or higher desktop GPUs along with special streaming software integrated into GeForce experience to stream games from the PC to SHIELD over the user's home Wi-fi network with ultra-low latency. Gamers use SHIELD as the controller and display for their favorite PC games as well as for steam big picture. This enables gamers to get a GeForce GTX PC gaming experience anywhere in their home Wi-fi network.

So i'm pretty sure you have to be at home to play it... if i could be somewhere else and play, it ~might~ be worth that $300