When does the Mac App Store produce notifications? According to Apple's page on Mountain Lion, the Mac App Store is supposed to send you notifications:

Mac App Store notifications
The Mac App Store notifies you when
updates are available for OS X or purchased apps.

This doesn't seem to be the case for me, as I have now opened the App Store twice to find updates (one for Day One, another to Xcode), having never received a notification for either. Does the App Store only notify about OS X updates, does this feature not actually function, or am I missing something?
You need to have it set to "Automatically download system updates" in the software update panel of system preferences. I got a notification about the xcode update today.