Previewed at WWDC, launching in the fall.
Facebook Rolling Out New iOS Photo Sync Feature
TechCrunch has more on how it works:
Here’s how Facebook Photo Sync works, at least for those lucky enough to be in the test. You go to the Photos app in the Facebook For iOS app’s sidebar navigation or the photos section of your Timeline and scroll to the bottom. There you may find an option to check out Photo Sync and then enable it, as seen in this photo sent in by tipster YoloFahad (an Instagrammer-extraordinaire).Facebook's support site has much more about Photo Sync, as well. Facebook is using background uploads within the Facebook iOS app to move the photos, not anything special with location APIs or Apple's iOS 6 Facebook integration.
Facebook will then automatically upload photos you shoot to an unpublished album only you can see. From there you can authorize which photos you want posted.
The app is very clear that all photos uploaded automatically are marked as private and cannot be viewed by others until specifically shared by the user. Users get 2 GB of complimentary photo storage.