You can learn a little more from:
http://www.devx.com/security/Article/17483

but it is a hosted service which Microsoft sells. Personally, I think
Passport has been a failure for Microsoft and is only useful to it on it's
own sites (hotmail/msn/...).  Here's an entry from wiki

"Having failed to gain market acceptance, Microsoft has recently stated they
will stop usage of Passport for entities outside of its own operations.
Despite media reports that Microsoft is phasing out the technology,
Microsoft rolled out a major update to the Passport service in May, 200"

So I'm not even sure if you can still buy it as a service...

Karl