@littlemouse I personally wrote it, and it has been the way I sign on to my own forum for over a year. http://vanillaforums.org/addon/proxyconnect-plugin
The problem was unintentional and unforeseen. You seem to be representing yourself as someone who knows something about software, so I'll give you the rundown.
SSO works by querying the master identity store (in this case cartalk.com) on behalf of the accessing user (you) and analyzing the response. If the accessing user is logged in to the master store, the SSO will receive basic account information such as Username, UserID, Email address. It will use these to either look up the corresponding account on the forum or, in the case of a user who is new to the forums, create a new account with those details and link to it. Then log in this account.
So we understand the basic idea: ask cartalk.com if the current user is logged in, and use the response to automatically log them in to the forum if they are. The problem is that if you think about this, it effectively doubles the load on both the forum and cartalk.com by causing a constant backchannel of "logged in? no? ok." "logged in now? oh, ok." "AND NOW?! ok :(" questions for every single user on every single page request.
The solution to this obvious problem is to make the SSO only ask that question when needed. Such as 1) the first time you open the community or 2) when you press the sign-in button on the community discussions page. This works seamlessly and invisibly for 99% of cases, and cuts down on all the excess wasted requests to and from the master store. Amazing!
If you think about this, the outcome is that the first time you arrive at cartalk.com after opening your web browser on a given day, the site loads and the Hot Discussions are fetched. Because this is coming from your browser and not cartalk.com, it actually triggers the first condition of the SSO: the first time you open the community. Obviously you are not logged in yet, and so the SSO goes: "ah, not logged in. we'll just wait till they decide to log in and press the 'sign in' button". Result: when you do decide to visit the forum, you are not logged in because the SSO has already tried and failed.
Our fix was to make the Community link from cartalk.com forcibly trigger the SSO question always (simulating an unpolluted 'first visit' condition when that link is clicked). A URL for that already existed, but I wrote some extra code anyway to account for the forcible nature of these new requests and prevent theoretical cases where this behaviour could cause an infinite loop for redirection. We figured that may annoy people.