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?

When new updates are available on the Mac App Store, it doesn’t notify you at first. It waits “for a while” and then downloads them in the background. This includes ALL updates. System updates, app updates, all of them. When the download is complete and ready to be installed, that’s when it notifies you.

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.