I have much the same question, although your answer isn't an option in my case unfortunately.
I am retrofitting a forum to an existing site with thousands of registered users and the site owner (my client) will not allow me to switch the login to the forum one. Only his registered site users will be allowed to access the forums (no problem there) but I must get around your forum login somehow. By this I mean that the user should only have to login a single time and that already happens on the main website. If tracking a date and time of login in the forum database is needed, that shouldn't be a problem.
I had the idea that when users register on the site, I could create a Stored Procedure to insert the relevant details into the relevant forum database tables (if needed) and then auto log them in when they connect to the forum home page, thus negating the need for a separate registration page to be completed and obviously, a second login.
I understand the 'security' answer given in the previous post, this is not an issue as the site's login system is also very secure.
I'm using the latest SQL Server version of the forum and it has it's very own database too.
Advice, hints and tips would be greatly appreciated as to how I can achieve this objective.
Cheers!