Maintaining state across mulitple domain

[Anon]

Hello,

I am wondering if anyone out there has successfully been able to create a system that allows for a user to maintain state across several domains.

For example, a user could login at mysite1.com and be able to surf to mysite2.com and still be logged in to the system.

I guess this would be similar to MSN's passport system, with the 1 login ability.

I'm looking for a multi-domain solution but I am also curious about any current methods of doing this with subdomains.

I know this all has to do with cookies but how do I allow a cookie to be read by several domains?

I know creating a cookie for each domain at one time might work but if there is any other solution i'd love to hear it!

Thanks in advance,
Kevin

[Anon]

If all these domains are being hosted on the one server (or at least, all using the same /tmp directory for storing session data), then session variables may work (I've never tried it though, so insert appropriate caveats).