Cydiablog has posted a video showing the leaked iPhone 5 back cover. The video was taken by eTradeSupply and they noted the following about the new unibody back cover:

• The next generation iPhone will be the same width as the current 4 and 4S models, but will be taller in size – around 4-inches diagonally.
• The upcoming iPhone will be much thinner. The exact dimensions are unknown, however.
• The leaked part is neither a battery door nor a middle plate. It is a single integrated part. The iPhone 5 back plate is one piece anchored by an aluminum alloy. Major design changes in the upcoming iPhone.
• Earphone jack has been moved from top to the bottom of the iPhone.
• Apple has redesigned the dock connector which is smaller in size. SIM card tray is different in size – a smaller one. So, expect a smaller SIM card tray.
• Enhanced speaker grills. If you carefully examine the video, you’ll notice an extra space at the bottom – may be to enhance the speaker’s quality.