Trying to maintain a login across multiple domains

Hi again, so I looked at the links you guys sent me in the other post, but I think this warrants a new one (thanks for all the responses btw!). So I'm trying to build a "system" to keep people logged in across multiple sites. It will be similar to google checkout, or microsoft passport, in that a third party is hosting the data, but its different in that the login will need to occur in the beginning/middle of a checkout process, and it will also need to keep the login persistent across multiple sites. In other words, someone would go to a site and they would be logged in, and then they could go to another site, and they would be logged in there as well. I read something about using cross domain cookies, but it looks like that would require modifications to the Apache config in the client sites. The problem is that I will not be able to mess with the "client" sites. Is there any way to do this in a purely web based (I assuming I could create a toolbar to install in the web browser, but I'm trying to keep this process non-invasive to consumers). Thanks in advance!

<no clue if this would work>
maybe if all the sites used the same database to hold the session data...

There is no way to do this without having access to all websites to modify their login mechanisms. Passport websites have a trust relationship with the authority that uses them, and unless you get all of the client websites to use a common login API, then it just wont work.