What's a MacBook Air to do in the case of a software emergency without an internal or external optical drive? Well, Apple's thankfully considered that situation, and has included a Software Reinstall USB drive with both the 11-inch and 13-inch models. As a bonus, it also makes an attractive addition to a keychain or necklace.
Update: We took some shots of the drive. It's a really small USB drive! Storage comes up at around 8GB, of which 7.5GB is used for the OS X and iLife installers.