Kellan Elliott-McCrae: "You can already specify that you want the output format of a Flickr
API call to be REST (POX), XML-PRC, SOAP (shudder, not sure that one
still works), JSON, or serialized PHP. We always wanted to support
formats like KML, or Atom but we were never quite sure how to represent
the results of a call to flickr.photos.getInfo() or flickr.photos.licenses.getInfo() as a KML.
Last week we finally got around to pushing out our 80% solution — an experimental response format for API methods that use the standard photos response format that allows you to request API responses as as one of our many feed formats.
You can now get the output of flickr.photos.search(), or flickr.favorites.getList() as Atom, or GeoRSS, or KML, or whatever.
The syntax is "&format=feed-{SOME_FEED_IDENTIFER}" where the feed identifiers follow the same convention you use when fetching…feeds."