It looks like NVIDIA is trying to get more people interested in its Shield portable Android gaming system. While it is a cool device, its really has not caught on all that well. To help gain attention and hopefully sales of the devices it looks like NVIDIA has worked with Valve to bring both Half-Life 2 and Portal to the Shield.
About a week ago NVIDIA sent out some cakes to a few Android and PC gaming websites as a hint that Portal will be coming to Android. It was confirmed that Portal will hit the Google Play store on May 12th with a pricetag of $9.99. The game will be playable on other Android devices other than the Shield. Just recently NVIDIA sent out a green crowbar to many sites giving the hint that Half-Life 2 will be coming to the Shield as well.
Of course this more or less confirms that Half-Life 2 will be coming to Android very soon. We do not have an exact date or price, but expect it to come shortly after the release of Portal.