There are several ways:
1. Boot to the recovery partition and reinstall (assuming original drive).
2. Clone the drive when system is new and save it.
3. Apple has restore flash drives available through AppleCare for disaster recovery.
4. You can buy a Lion flash drive.
5. If you have another Mac and upgrade it to Lion you can made an install DVD from the Lion installer download.
In addition, expect at some point the Internet recovery feature will be available on new iMacs (and MacBook Pros and Mac Pros) the same as it is for 2011 MacBook Airs and Mac minis.