Just confirmed from the earnings call: OS X Lion will launch tomorrow, July 20. This was an unusual way to announce the launch, but it confirms the week's rumors.
Lion will be available exclusively via the Mac App Store. There's no word yet whether it will be a rolling international launch across time zones (beginning in New Zealand and ending in Hawaii) or if the launch will occur simultaneously worldwide. Of note, it is already July 20 on half of Earth, but Lion is still not available.
We'll update this post later if we get more information.