Yes the installer is updated with 10.8.x releases as they come out. You will need to update the installer app through the App Store as you would for any other app. ( Source: OS X 10.8 Mountain Lion: the Ars Technica review).
Like any other application purchased from the Mac App Store, these OS
installers are updated in-place as part of the normal Mac App Store
update process when new versions are released. It's as if your Mac OS
X 10.6.0 installation disc gradually morphed into a 10.6.1 installer,
then a 10.6.2 installer, and so on.
Example given is 10.6, but the reviewer is talking about 10.8.