Apple's yearly developer conference should see iOS 8, OS X 10.10, and likely some hardware.
Mid-2007 Prototype MacBook Air Found on eBay
After receiving the unit, Brett33 noticed that the bottom of the laptop was made of black aluminum and that the screen bezel did not have "MacBook Air" written on it. The laptop runs an interim build of Mac OS X 10.5 (8R4016).
No "MacBook Air" wording on Bezel
Black bottom (removed)
The serial number on the machine indicates that it was built in May 2007, over six months before the MacBook Air was introduced. Another poster was able to specify that this model represented "M82 PROTO2 ENC 4", based on the serial number.
Though rare, prototype Apple machines have been known to circulate.