According to AppleInsider's sources, this flickr image is the closest mockup of the Apple Tablet we've seen to date. And yes, it's essentially a big, first-gen iPhone that's "met its match with a rolling pin".
Rather than a plastic back, AI claims the Tablet will have the same, two-tone aluminium look of the original iPhone. But while this image is thought to be close to Apple's final design, it doesn't sound like a full-fledged OS X will be on board. Why? The Tablet will have a Home button (along with a 10-inch screen, 3.5-mm stereo headphone jack, built-in speaker grills, a microphone, GPS, 3G connectivity and a 30-pin dock connector, if AppleInsider has it right).