Adobe Camera Raw 8.5, has a fantastic new addition to the Graduated and Radial Filters – a brush to selectively hide the effects! This first illustration is the original image.
In the image below, a Graduated filter was added to darken the sky, however the effect is also applied to the top of the mountains because they are also affected by the Graduated filter.
To remove the effect in the top of the mountains, with the Gradient Filter selected, choose the Brush option. The Brush options include Size, Feather and Flow as well as Auto Mask (used to automatically detect edges based on contrast and color) and Clear (to remove Brush overrides).
This last image shows the result from using the Graduated Filter Brush to paint out the effect in the mountains while still retaining the effect in the sky area.
And one more super shortcut – to keep the Graduated and Radial filters eraser size the same as the brush, click the flyout menu (to the right of the Graduated Filter panel header), and toggle “Separate Eraser Size” from the menu.
As mentioned here, updates to Camera Raw 8 for Photoshop CS6 only include new camera support, lens profile support, and bug fixes. The new features listed in the release notes are only available in Photoshop CC.