EDIT: Running on 3 hours sleep doesn't help me! Naturally if you're not going to distribute Lion with the machine don't follow this procedure, and restoring from the SL USB key is the best bet.
If you have a USB disk you don't mind partitioning, make a partition ~ 4GB in size on it using Disk Utility. The USB disk itself should be bootable (GUID partition table). You then need to find the InstallESD.dmg from the Lion installer app that you downloaded from the MAS (go to the app in your Applications folder, right-click, Show Contents, navigate into the SharedResources folder). Mount that sucker by double-clicking it in the finder (it may take a little bit to verify the image). You'll have one new disk image showing in Finder called "Mac OS X Install ESD".
Back in Disk Utility you need to click on the partition you made on your USB disk. Click on the Restore tab on the right side. Drag the
Mac OS X Install ESD partition from the left pane to the
Source box. Then drag the partition you made on your USB disk to the
Destination box. Click the Restore button and go have a beer for the next few mins. If all goes well the Lion installer will be living on your USB disk. Mount it in Disk Utility by highlighting the partition you made and clicking the Mount button at the top (if it's not mounted already). To no one's surprise, that partition should now be called Mac OS X Install ESD.
Now that you have your bootable Lion install media, plug that sucker into the Air you want to reset. Powerup or reboot the Air so it can boot off the USB disk. As the machine is booting up -- when you here the chime -- press Option. You should be presented with a list of devices from which to boot and your USB disk should be one of them (I think it calls it simply
Mac OS X but the icon will be a USB disk icon). Double-click the USB disk.
You'll be brought into the Lion Recovery Utilities. Here you can do things like fire up Disk Utility, terminal, etc, and Reinstall. You will want to use the Disk Utility to reformat the Air's SSD and then do the OSX reinstall.
Hope this helps a bit. I wrote this very quickly from memory and I know there are other guides out there but I'll leave that for Google and Mroogle to answer.
Cheers & good luck