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In this screen shot, taken from an Amazon demo at PDC 2003, Jim Allchin showed off a rather flashy Amazon browsing experience, which utilized the preview pane and one of many 3D views. (To note, this demo was meant to also show off some of the underlying services that were being baked into Longhorn.) The theme you see here is Slate, which was rather hideous compared to the beautiful Aero theme showcased earlier. It was merely temporary.
Here's an image that showcased Phodeo's 3D view from a different perspective.
Yet one more of the myriad of Longhorn mock-ups created, this one showcases thinking for a sidebar/taskbar combination. Personally, I wasn't a fan of it, but the window that's open, however, shows yet another take on how beautiful the preview pane could be made to look and function. (Thanks to Long Zheng for initially sharing this image on his blog.)