Flickr have announced the end date for the old authentication method.
After the 31st July you will no longer be able to use the old authentication methods, and will have to use the new OAuth 1.0a authentication flow in your applications.
http://code.flickr.com/blog/2012/01/13/farewell-flickrauth/
I’m glad to say that Flickr.Net library already fully supports OAuth, and the old methods are deprecated. I will be deleting these methods from the library nearer the time in an effort to get everyone to move forward with the new OAuth flow.