Lion doesn't come on any physical media.  Instead, you have download it from Apple's servers from the Apple store.  What if you bought a Mac with Lion already installed and you desperately need the install DVD because your hard drive got hosed.  The following will give you instructions on how to burn your own install DVD.

1. Download Lion from the Mac App Store. The installer should show up in your Applications folder.
2. Right-click on the installer and hit "Show Package Contents". Navigate to Contents > SharedSupport and look for a file called "InstallESD.dmg".
3. Open up Disk Utility and drag the DMG file into the left-hand sidebar. If you're burning it to a DVD, insert your DVD, select the disk image in the sidebar, and hit the "Burn" button. Skip down to the last step to use it.
4. If you want to burn Lion to a USB flash drive, plug it in and click on it in the left-hand sidebar in Disk Utility. Go to the Partition tab and select "1 Partition" from the dropdown menu. Choose "Mac OS Extended (Journaled) on the left.
5. Hit the Options button under the partition table and choose "GUID Partition Table". You'll need this to make the drive bootable on a Mac. Hit the Apply button when you're done to format your drive (note: it will erase everything on the drive).
6. Click on the "Restore" tab, choose the InstallESD.dmg file as the source and your flash drive as the destination. Hit the Apply button and it will create your bootable USB drive.
7. Reboot into OS X and hold the option key when you hear the startup chime. You can boot into your DVD or flash drive from there.