When Apple release iLife '06 it included a "feature" in iPhoto called photocasting (similar to the all successful podcasting). So basically I'm guessing it's just iPhoto subscribing to some rss feed right? I was initially interested in how this worked, so I've had a quick google search to see what sorts of things come up. Here the summary of a user that was posted on ones of Apple's mailing lists:
"To sum up, the "photocasting" feature centers around a singleundocumented extension element in a namespace that doesn't need to bedeclared. iPhoto 6 doesn't understand the first thing...