I’ve received several messages asking about this in the last few weeks, so that means it’s time to write it up!
You may have at some point seen a notification badge show up on your Aperture icon, like you’d see on Mail or Messages, and wonder what it’s for. The simple answer is it’s from a Shared Photo Stream. Whenever someone “likes” or comments on a photo you’ve shared with them, you’ll get a notification anywhere you’ve enabled Photo Stream. That could be any iOS device on iOS 6, or it could also be in iPhoto or Aperture.
When you see the badge, if you open Photo Stream in Aperture you should see a blue dot on the Shared Photo Stream that has a notification, and if you open that, you’ll see blue dot(s) on the photos that have new likes or comments.[more]