During the webcast series I did over the past few weeks, I showed a demo of a lab that covers Windows CardSpace.
In the last part of the lab we show one example of a server control that does just that. You can actually get the source code for that CardSpace server control by launching the lab and copying it out. Also, I found a few more out on the Web that you might want to consider, like this one by Dominick Baier and this one on the netfx3.com site.
I've been thinking about integrating Cardspace into a project I'm working on...this will make it all the more simple when I get to that...Thanks, Glen!