I've noticed this morning that app updates stopped showing up in desktop iTunes on Mac and Windows. It's interesting that the App Store on my iPhone, iPod touch and iPad devices still notifies me of available updates like before. For instance, tapping the Updates tab in the App Store storefront app on iDevices shows all of the available updates.
Not so much in desktop iTunes, unfortunately.
Individual app updates are still available by visiting an app's page in iTunes and hitting the Update button, they just won't show up in the Updates tab section section. Twitter says a growing number of users are now experiencing this hiccup, with early reports of the issue dating as far back as a week ago...