Flickr announced today that all new users to Flickr will be created with Yahoo accounts. People with existing Yahoo accounts can create their Flickr username just by logging into the site and people with existing Flickr accounts can link their Flickr and Yahoo accounts.
What this means for you, the developer. If you're using the new authentication API, not much at all. If you're still using the old, depreciated methods, new Flickr users (and old users who link their accounts) will not be able to log into your apps. This is the time to update your application before they shut it off forever.