For the last six days, app updates have not been showing up in iTunes on the Mac and on the PC for many people around the globe. I can confirm that app updates have stopped appearing in iTunes on my Mac (while those missing updates still appear in the App Stores on my iPhone and iPad). The problem seems to have started last Tuesday. Since then, no updates have appeared in iTunes.
Not all of us at TUAW are experiencing the loss of functionality of app updates in iTunes, but there is currently a thread in Apple's discussion forums with more than 120 replies of people stating that app updates have stopped appearing in iTunes on both Macs and PCs. It's not clear why those affected are and others are not. Reports of missing updates are coming in from people in multiple locales and with various country-specific iTunes accounts (such as the US and the UK).
Users who are afflicted can still get app updates on their iOS devices, but in order to get those updates on iTunes (or update apps they may have in iTunes, but not on their iOS devices) they must specifically seek out that app in the App Store and click the Update button to download it.
There's no word from Apple on how widespread the problem is, but reports of the problem are also starting to appear on Twitter.