Apple today released OS X 10.8 Mountain Lion on the Mac App Store and has now also removed OS X 10.7 Lion from the Mac App Store. You can no longer go to the Lion page via its direct link, by searching or by finding it on the top grossing list as it was earlier this morning.
Apple presumably pulled Lion because users wanting to upgrade to Mountain Lion can also do so from Snow Leopard. Keeping both Lion and Mountain Lion on the Mac App Store would have presumably caused confusion for some users - particularly given Lion cost $29.99 and Mountain Lion costs just $19.99 - seemingly suggesting that Lion was the latest OS.
Update 4:05 PM: Customers who purchased Lion should still be available to download Lion from the Mac App Store when you option click the Purchased tab.
You can read the MacStories review of Mountain Lion here.