Starting with OS X Lion, Apple's Mac App Store became the primary means for distributing OS X, but the version of OS X in the store would sometimes lag behind the latest release. With Mountain Lion, the App Store is now being used to distribute system updates in addition to the full OS installer, and along with this development Apple appears to be updating the full OS download version concurrently with the release of the latest patch.
This means that if for some reason you are looking for an older release of OS X than the most current version, you will not find it in the Mac App Store. Though in general, it's a good idea for most people to keep their OS up-to-date for security and stability reasons, there may be times when, in troubleshooting compatibility issues, someone wants to install the first version of OS X and upgrade to a specific OS version prior to the most current release.
By no longer supplying OS X 10.8.0 and keeping the OS installer up-to-date, Apple has made such troubleshooting approaches more difficult. Therefore, if you think you might need to use OS X in this manner, you might considerand using that to preserve a relatively early release of the OS that can be used for reinstallation.
Though Apple is now keeping the Mac App Store version of OS X up-to-date, and it provides information about the latest patch on its Mountain Lion App Store product page, so far only the full installer for OS X is downloadable through the store, with no option there for grabbing a standalone updater. However, as with previous updates to OS X, Apple is still providing updater downloads through its support Web site, so you can still get the standalone installer for OS X 10.8.1 if needed.
Since this latest update is the first for Mountain Lion, Apple's support downloads page has only the one delta updater. As future updates to OS X are released, expect Apple to continue providing both delta and combo updaters through the Support downloads page in addition to keeping the App Store offerings current.