Apple has removed Lion from the Mac App Store. This means that it’s no longer available for download. Those who purchased it can still see it on their Purchases tab in the Mac App Store, but are reportedly unable to download it.
That said, users who have to reinstall will have to do so with Apple’s online recovery (Lion Internet Recovery). This should work for users who are registered Lion owners.
One question that some are asking is whether they can upgrade if they made the purchase and have additional computers that haven’t had Lion Installed yet. We will just have to see what those users experience.