Well, you'll have to do some work to make this happen. Microsoft Passport does cross-domain authentication through redirects. Say you login to Hotmail. You get redirected first to passport.com, authenticate, then get redirected back to Hotmail with a coded identifier. Hotmail can then make a request of passport.com (e.g., validation of the identifier which will yield a user id).
If you visit Expedia or Investor or other MSFT sites, you will get redirected momentarily to Passport to see if you have a passport cookies. If so, it will know who you are and you won't have to login again.
If you want to emulate this type of model - admittedly complex - it will give you cross-domain authentication.
Dave
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