How to format the OS X installer USB stick that comes with MacBook Air?
From Apple's MacBook Air: Frequently Asked Questions about Software Reinstall Drive:


  Note: The MacBook Air Software Reinstall Drive is read only.  You cannot erase it, reformat it, or reuse it as a general purpose USB storage device.


Honestly, thumb drives are dirt cheap these days—why would you want to lose your only way to reinstall your OS?