Apple has apparently dropped OS X Lion from the Mac App Store. Those who have purchased it are still seeing it in their purchases list, though some TUAW staffers are saying they're not able to download it. I wasn't able to download it on my MacBook Pro, though I was able to start doing so on my iMac.
Those with OS X Lion who are not making the Mountain Lion upgrade should be able use the Lion Internet Recovery process if there are any issues or use Lion Recovery Disk Assistant to make a backup on an external drive. We also recommend making a bootable backup of your computer whether you're sticking with Lion or upgrading to Mountain Lion.