I'm interested to know if there's any sites out there that has implemented CardSpace as an alternative authentication. I certainly like to see how different it is from, say, using OpenID.
One example of someone using information cards is JanRain, an OpenID provider, allows the use of information cards for authentication to its myOpenID service
It should also be noted that OpenID is for identity, and leaves the authentication component up to the providers. Some providers use a simple username/password, while others offer a more secure solution such as information cards or the ImageShield technology used by Vidoop.
Information cards / CardSpace and OpenID are not mutually exclusive systems.