Can somebody help me find this wallpaper (image taken from http://www.apple.com/ipad-mini/). I've looked all over the internet but couldn't find it.
Apple sourced this image from Robbie George Photography. It's called "Lake Mattamuskeet", you'll find it on his page.
One way it could be done is to take the image you posted, and using Photoshop or other graphics editing app, magic-wand select the image on the iPad, copy and paste into its own layer, select it again and use the Edit menu >Transform > Skew to square the corners. Then save the now relatively rectangular image as a JPG or whatever and use that as your background pic.
Go to Google images and click on the camera icon:
Upload the file, and see what google finds for you. With luck, you'll find a hi-res royalty-free image.