Reports across the Internet suggest that iPhone 5 boxes have left the factory gates and are now en-route to customers. Some have reported getting shipping dates as early as today, 17 September 2012.
9to5Mac.com suggests that the iPhone 5 units have shipped from the Zhengzhou region of China, possibly from the nearby Taiyuan Foxconn Factory.
See: iPhone 5 review
However, just because the iPhone 5 has shipped early, does not mean you will necessarily get it prior to the 21 September 2012 launch. UPS may be under instruction from Apple to hold the parcels and deliver them all to customers on Friday. However, we wouldn't be surprised if somebody manages to get hold of theirs a day or two early.
Sign in to the Apple store to see if your iPhone 5 has shipped early.