Here's what I did:
- Using any mac, format a usb flash drive (at least 8 gb) as journaled extended.
- Connect the usb flash drive to your mba and boot into the recovery partition (hold cmd-R at startup).
- Choose "Reinstall Mac OS X".
- Choose to install it on the usb flash drive.
- The installer goes through two steps: 1) downloading Lion and 2) installing Lion. The progress bar will reset after it's finished downloading and when it starts installing. When it starts installing, power off the computer.
- Disconnect the usb flash drive.
- Boot back into your regular Lion partition on the mba.
- Connect the usb flash drive, and you will have access to InstallESD.dmg.
Note that this process leaves your recovery partition in a weird state - you will need to reset your pram after doing this, or it will forever keep trying to finish the install to the usb flash drive next time you boot into it.