Create a new Alpha Channel in the Channels Palette. Go to Filter>Render>Clouds. Hit Command+T and scale the clouds up to about 180%. Load the selection of the Channel by holding Command and clicking on its thumbnail. In the Layers Palette, create a new Solid Color adjustment layer just above the background. Make the fill 50% gray (#808080). Blur the cloud mask with a 30px Gaussian Blur.
Just above that layer, create a Gradient Adjustment Layer and set up the gradient to go from transparent from the top-left to white at the bottom-right.