Although it's (weirdly?) not a "featured app," OS X Lion is indeed now available for download from the Mac App Store. The 3.5GB download will cost you $30, and upgrades your Mac with over 250 new features—some of which you may be just as happy without.
Update: Okay, now it's the featured app. Can't miss it. Giant banner on the front door, the one with a Lion on it. And here I was worried Apple was selling themselves short! [Mac App Store]