Apple's new online distribution for its operating system provides OS X in a disk image format that is stored on Apple's distribution servers. What this means is that unlike optical media that is a set version and cannot be updated, the version in the Mac App Store can be updated if Apple just replaces the installation image with an updated version. Recently this happened with the initial 10.7.0 version in the Mac App Store being replaced with the recently released 10.7.1 version.
Given Apple's online installation options for OS X, this has the benefit of allowing you to install the latest version of the OS in one step. In the past when you wanted to reinstall your OS, you needed to install the version that is on your installation DVD, and then download the latest combo updater to get your version to the latest one.
This procedure worked well, but it took additional steps and was a little cumbersome. Now if you wish to reinstall, the OS X installer should provide you with the latest version every time. Just boot off the OS X restore volume (be it an internal partition or external drive), and if you choose to reinstall OS X then the latest version will be downloaded and applied.
Unfortunately, though this is convenient, it also means that older versions will not be available, but there may be situations for some where reinstalling an older version of the OS may be desired. If you are concerned about needing an older version of Lion, then you have a couple of options to retain them:
The initial download from the Mac App Store was version 10.7.0, so if you downloaded that version of OS X and have retained the installer, then you should be able to use that to install to any supported Mac. From here you can access Apple's Support Downloads to get the Combo updaters Apple has made available, and use them to upgrade to any subsequent version of OS X. This method is similar to the previous options people had for reinstalling OS X followed by updating.
Download installers each time
The second option is to take advantage of your current license for OS X, and redownload the updated Lion installers from the Mac App Store when they get updated (press the Option key when clicking the "Purchases" section of the store to redownload the installer). From here you can either create an archive of them on a disk, or use our instructions for creating standalone installation disks from them. If you do this and create your own installation drives, you can just maintain one and keep it updated to the latest version, or you can create a new one for every release of OS X if you wish.
Regardless of the approach you choose to maintain or update your Lion installation media, do keep in mind that once Apple updates the Mac App Store version, then the prior version will be gone. Right now that means there is no way (beside possibly purchasing Apple's USB Lion installer drive) to get version 10.7.0 for your system, and when OS X 10.7.2 is released then the same will be true for both 10.7.1 and 10.7.0.