Apparently the developer previews of Lion (via the App Store) download a disk image with the installer on. It's not known yet whether you can burn the disk image as a bootable DVD but you should at least be able to back it up to external media for re-installation.
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Looks like you can create a bootable DVD or USB dongle from the downloaded Lion installer.
http://holgr.com/blog/2011/02/creati...0-7-lion-disc/