How to re-download OS X Lion from 10.6.8
When a new OS X is released, it isn't no longer possible to download the previous version(s).

Apple has however now with the release of OS X Mavericks (10.9) again opened up for re-download of Lion and Mountain Lion (if previously purchased) - or to purchase the two OS's for a price of $19.99 each.

OS X Lion (10.7)

OS X Mountain Lion (10.8)