Good in-depth interview by Richard Koman with Stuart Butterfield, one of the masterminds behind Flickr.
Choice quote on APIs by Butterfield:
On the strictly practical side, I think we had one person inquire about using the SOAP version of the API. I don't know if any apps were actually built. There is at least one application built on XML-RPC. But all the others--I don't even know how many there are--are built on the REST API. It's just so easy to develop that way; I think it's foolish to do anything else.