According to 9to5 Mac, hidden inside that updated file system is a "Media Stream" folder, containing a "Photo Streaming" folder that contains strong evidence that it will allow iPhone users to let friends subscribe to their collections of photos, perhaps residing in the cloud.
There's further evidence of social media capabilities, including privacy preferences that would let the user decide who would be allowed to follow the streams, and graphics inside the operating system for the "Photo Stream" feature.
Beyond that evidence, when Gizmodo's Rosa Golijan was testing iOS 4.3 Beta 2, she found what she called "a mysterious error message" using the term "Photostream," as you can see in the graphic above. She also noticed an invitation system, and a connection to Mobile Me.
This isn't the first evidence found for social media features in this iOS 4.3 developer build. Just last week, a "Find My Friends" feature was revealed, along with AirPlay video support for third party apps and personal hotspot support for the iPhone.
It was unclear when any of these features would be appearing in Apple's iOS.